Fix tests for .NET Framework environment variable behavior

- Moved empty string environment variable tests under #if NET conditional
- On .NET Framework, SetEnvironmentVariable("", "") deletes the variable
- Tests now only validate empty string semantics on .NET Core/.NET where supported
- Core tests (non-empty values) run on both .NET Framework and .NET

Co-authored-by: baronfel <573979+baronfel@users.noreply.github.com>

Author: Copilot

src/Build.UnitTests/ConsoleLogger_Tests.cs

@@ -1965,7 +1965,6 @@ internal bool IsRunningWithCharacterFileType()
 
         [Theory]
         [InlineData(null, null)] // Default: colors enabled
-        [InlineData("", null)] // NO_COLOR="" (empty string) means colors disabled (variable is set)
         [InlineData("1", null)] // NO_COLOR set, colors disabled
         [InlineData("1", "1")] // NO_COLOR supersedes FORCE_COLOR
         public void ColorEnvironmentVariables_DisablesColor(string noColor, string forceColor)
@@ -2011,9 +2010,38 @@ public void ColorEnvironmentVariables_DisablesColor(string noColor, string force
             }
         }
 
+#if NET
+        // On .NET, empty string environment variables are supported
+        [Theory]
+        [InlineData("")] // NO_COLOR="" (empty string) means colors disabled (variable is set)
+        public void ColorEnvironmentVariables_DisablesColor_EmptyString(string noColor)
+        {
+            using (TestEnvironment env = TestEnvironment.Create())
+            {
+                env.SetEnvironmentVariable("NO_COLOR", noColor);
+
+                EventSourceSink es = new EventSourceSink();
+                SimulatedConsole sc = new SimulatedConsole();
+                ConsoleLogger logger = new ConsoleLogger(LoggerVerbosity.Normal, sc.Write, sc.SetColor, sc.ResetColor);
+                logger.Initialize(es);
+
+                // Log an error
+                es.Consume(new BuildStartedEventArgs("Build started", null));
+                BuildEventContext context = new BuildEventContext(1, 1, 1, 1);
+                BuildErrorEventArgs errorArgs = new BuildErrorEventArgs(null, "ERR001", "file.cs", 1, 1, 0, 0, "Test error", null, null);
+                errorArgs.BuildEventContext = context;
+                es.Consume(errorArgs);
+                
+                string log = sc.ToString();
+                log.ShouldNotContain("<red>");
+                log.ShouldNotContain("<yellow>");
+                log.ShouldNotContain("<green>");
+            }
+        }
+#endif
+
         [Theory]
         [InlineData("1")] // FORCE_COLOR set, colors enabled (will use ANSI codes)
-        [InlineData("")] // FORCE_COLOR="" (empty string) means colors enabled (variable is set)
         public void ColorEnvironmentVariables_ForcesColor(string forceColor)
         {
             using (TestEnvironment env = TestEnvironment.Create())
@@ -2042,6 +2070,36 @@ public void ColorEnvironmentVariables_ForcesColor(string forceColor)
             }
         }
 
+#if NET
+        [Theory]
+        [InlineData("")] // FORCE_COLOR="" (empty string) means colors enabled (variable is set)
+        public void ColorEnvironmentVariables_ForcesColor_EmptyString(string forceColor)
+        {
+            using (TestEnvironment env = TestEnvironment.Create())
+            {
+                if (forceColor is not null)
+                {
+                    env.SetEnvironmentVariable("FORCE_COLOR", forceColor);
+                }
+
+                EventSourceSink es = new EventSourceSink();
+                SimulatedConsole sc = new SimulatedConsole();
+                ConsoleLogger logger = new ConsoleLogger(LoggerVerbosity.Normal, sc.Write, sc.SetColor, sc.ResetColor);
+                logger.Initialize(es);
+
+                // When FORCE_COLOR is set, logger uses ANSI codes which write directly to Console.Out
+                es.Consume(new BuildStartedEventArgs("Build started", null));
+                BuildEventContext context = new BuildEventContext(1, 1, 1, 1);
+                BuildErrorEventArgs errorArgs = new BuildErrorEventArgs(null, "ERR001", "file.cs", 1, 1, 0, 0, "Test error", null, null);
+                errorArgs.BuildEventContext = context;
+                es.Consume(errorArgs);
+                
+                // Just verify no crash
+                sc.ToString().ShouldNotBeNull();
+            }
+        }
+#endif
+
         [Theory]
         [InlineData("false")]
         public void UseColorParameter_DisablesColor(string paramValue)

src/Build.UnitTests/TerminalLogger_Tests.cs

@@ -1139,11 +1139,9 @@ public async Task DisplayNodesRestoresStatusAfterMSBuildTaskYields_TestProject(b
 
         [Theory]
         [InlineData(null, null, true)] // Default: colors enabled
-        [InlineData("", null, false)] // NO_COLOR="" (empty string) means colors disabled (variable is set)
         [InlineData("1", null, false)] // NO_COLOR set, colors disabled
         [InlineData("1", "1", false)] // NO_COLOR supersedes FORCE_COLOR
         [InlineData(null, "1", true)] // FORCE_COLOR set, colors enabled
-        [InlineData(null, "", true)] // FORCE_COLOR="" (empty string) means colors enabled (variable is set)
         public void ColorEnvironmentVariables(string? noColor, string? forceColor, bool shouldUseColor)
         {
             using (var env = TestEnvironment.Create())
@@ -1164,6 +1162,32 @@ public void ColorEnvironmentVariables(string? noColor, string? forceColor, bool
             }
         }
 
+#if NET
+        // On .NET, empty string environment variables are supported and different from null
+        [Theory]
+        [InlineData("", null, false)] // NO_COLOR="" (empty string) means colors disabled (variable is set)
+        [InlineData(null, "", true)] // FORCE_COLOR="" (empty string) means colors enabled (variable is set)
+        public void ColorEnvironmentVariables_EmptyString(string? noColor, string? forceColor, bool shouldUseColor)
+        {
+            using (var env = TestEnvironment.Create())
+            {
+                if (noColor is not null)
+                {
+                    env.SetEnvironmentVariable("NO_COLOR", noColor);
+                }
+                if (forceColor is not null)
+                {
+                    env.SetEnvironmentVariable("FORCE_COLOR", forceColor);
+                }
+
+                TerminalLogger logger = new TerminalLogger();
+                logger.Initialize(_centralNodeEventSource);
+
+                logger.ShouldUseColor().ShouldBe(shouldUseColor);
+            }
+        }
+#endif
+
         [Theory]
         [InlineData("true", true)]
         [InlineData("false", false)]