Fix ASTHelpers#isRuleKind on JDK versions without CaseTree#getCaseKind

Fixes #4479 (comment)

PiperOrigin-RevId: 653615823

Author: cushon

check_api/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -2813,6 +2813,9 @@ private static Method getCaseTreeGetExpressionsMethod() {
    * <expression>}.
    */
   public static boolean isRuleKind(CaseTree caseTree) {
+    if (GET_CASE_KIND_METHOD == null) {
+      return false;
+    }
     Enum<?> kind;
     try {
       kind = (Enum<?>) GET_CASE_KIND_METHOD.invoke(caseTree);

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

@@ -50,7 +50,6 @@ public void positive() {
 
   @Test
   public void negativeStatement() {
-    assumeTrue(RuntimeVersion.isAtLeast14());
     testHelper
         .addSourceLines(
             "Test.java",