check if model id is null when undeploy (opensearch-project#2006)

Signed-off-by: Yaliang Wu <ylwu@amazon.com>

Author: ylwu-amzn

plugin/src/main/java/org/opensearch/ml/action/undeploy/TransportUndeployModelsAction.java

@@ -107,6 +107,10 @@ protected void doExecute(Task task, ActionRequest request, ActionListener<MLUnde
         String[] modelIds = undeployModelsRequest.getModelIds();
         String[] targetNodeIds = undeployModelsRequest.getNodeIds();
 
+        if (modelIds == null) {
+            listener.onFailure(new IllegalArgumentException("Must set specific model ids to undeploy"));
+            return;
+        }
         if (modelIds.length == 1) {
             String modelId = modelIds[0];
             validateAccess(modelId, ActionListener.wrap(hasPermissionToUndeploy -> {