Run JDK11 tests for KAPT only if JDK11 is available

Otherwise, just skip running them.

compiler/tests-common/tests/org/jetbrains/kotlin/test/KotlinTestUtils.java

@@ -605,11 +605,11 @@ public class KotlinTestUtils {
         return new File(jdk9);
     }
 
-    @NotNull
+    @Nullable
     public static File getJdk11Home() {
         String jdk11 = System.getenv("JDK_11");
         if (jdk11 == null) {
-            throw new AssertionError("Environment variable JDK_11 is not set!");
+            return null;
         }
         return new File(jdk11);
     }