NIFI-14472: Guard against null kubernetesClient in KubernetesConfigMapStateProvider.shutdown()

shutdown() was calling kubernetesClient.close() unconditionally. If the
provider was never initialized (e.g. in unit tests that only call
getSupportedScopes()), kubernetesClient is null and a NullPointerException
is thrown from the @AfterEach teardown.

Also null-guard the logger reference for the same reason.

Author: rakesh-rsky

nifi-framework-bundle/nifi-framework-extensions/nifi-framework-kubernetes-bundle/nifi-framework-kubernetes-state-provider/src/main/java/org/apache/nifi/kubernetes/state/provider/KubernetesConfigMapStateProvider.java

@@ -137,12 +137,16 @@ public void initialize(final StateProviderInitializationContext context) {
     }
 
     /**
-     * Shutdown Provider
+     * Shutdown Provider. Safe to invoke even when the Provider was never initialized.
      */
     @Override
     public void shutdown() {
-        kubernetesClient.close();
-        logger.info("Provider shutdown");
+        if (kubernetesClient != null) {
+            kubernetesClient.close();
+        }
+        if (logger != null) {
+            logger.info("Provider shutdown");
+        }
     }
 
     /**