fixing empty labels handling

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/ZeroShotClassificationConfig.java

@@ -233,7 +233,7 @@ public String getHypothesisTemplate() {
     }
 
     public List<String> getLabels() {
-        return labels;
+        return Optional.ofNullable(labels).orElse(List.of());
     }
 
     @Override

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/nlp/ZeroShotClassificationProcessor.java

@@ -74,6 +74,9 @@ public NlpTask.RequestBuilder getRequestBuilder(NlpConfig nlpConfig) {
         } else {
             labels = this.labels;
         }
+        if (this.labels == null || this.labels.length == 0) {
+            throw ExceptionsHelper.badRequestException("zero_shot_classification requires non-empty [labels]");
+        }
         return new RequestBuilder(tokenizer,  labels, hypothesisTemplate);
     }