[StatementSwitchToExpressionSwitch] Bugfix to not group case null with a normal case following it

PiperOrigin-RevId: 756958777

Author: markhbrady

core/src/main/java/com/google/errorprone/bugpatterns/StatementSwitchToExpressionSwitch.java

@@ -298,6 +298,14 @@ private static AnalysisResult analyzeSwitchTree(SwitchTree switchTree, VisitorSt
         return DEFAULT_ANALYSIS_RESULT;
       }
 
+      // Null case can never be grouped with a following case (except possibly default)
+      if (caseIndex > 0
+          && groupedWithNextCase.get(caseIndex - 1)
+          && isNullCase.get(caseIndex - 1)
+          && !isDefaultCase) {
+        return DEFAULT_ANALYSIS_RESULT;
+      }
+
       // Grouping null with default requires Java 21+
       if (caseIndex > 0
           && isNullCase.get(caseIndex - 1)

core/src/test/java/com/google/errorprone/bugpatterns/StatementSwitchToExpressionSwitchTest.java

@@ -1377,6 +1377,40 @@ public int foo(Suit suit) {
         .doTest();
   }
 
+  @Test
+  public void switchByEnum_firstNullCase_noError() {
+    // The null case cannot be grouped with a following regular case per Java syntax
+    assume().that(Runtime.version().feature()).isAtLeast(21);
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              public int invoke() {
+                return 123;
+              }
+
+              public int foo(Suit suit) {
+                switch (suit) {
+                  case null:
+                  case DIAMOND:
+                    return invoke();
+                  case SPADE:
+                    throw new RuntimeException("Spade");
+                  case HEART:
+                    throw new RuntimeException("Heart");
+                  case CLUB:
+                    throw new NullPointerException("Club");
+                }
+              }
+            }
+            """)
+        .setArgs(
+            "-XepOpt:StatementSwitchToExpressionSwitch:EnableReturnSwitchConversion",
+            "-XepOpt:StatementSwitchToExpressionSwitch:EnableDirectConversion=false")
+        .doTest();
+  }
+
   @Test
   public void switchByEnum_nullGroupedWithDefault_error() {
     assume().that(Runtime.version().feature()).isAtLeast(21);