Fix truthiness of Sequence toolchain variables

The truthiness of sequence toolchain variables has been flipped for a long time. This became a problem in 1b7dfd8, which normalized `null`/`None` values to empty sequences and thus ended up flipping the truthiness (e.g., when consumed via `expand_if_false`).

Closes #26568.

PiperOrigin-RevId: 785412332
Change-Id: I12e6d668c77d9557bd5d3a07fc55646c799f5816

Author: fmeum

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainVariables.java

@@ -617,7 +617,7 @@ public String getVariableTypeName() {
 
     @Override
     public boolean isTruthy() {
-      return values.isEmpty();
+      return !values.isEmpty();
     }
 
     @Override

src/test/java/com/google/devtools/build/lib/rules/cpp/LinkBuildVariablesTest.java

@@ -115,6 +115,13 @@ public void testLibrarySearchDirectoriesAreExported() throws Exception {
 
     ConfiguredTarget target = getConfiguredTarget("//x:bin");
     CcToolchainVariables variables = getLinkBuildVariables(target, Link.LinkTargetType.EXECUTABLE);
+    assertThat(
+            variables
+                .getVariable(
+                    LinkBuildVariables.LIBRARY_SEARCH_DIRECTORIES.getVariableName(),
+                    PathMapper.NOOP)
+                .isTruthy())
+        .isTrue();
     List<String> variableValue =
         getSequenceVariableValue(
             getRuleContext(),