test fix

Author: nikhil-zlai

online/src/main/scala/ai/chronon/online/fetcher/GroupByResponseHandler.scala

@@ -249,8 +249,8 @@ class GroupByResponseHandler(fetchContext: FetchContext, metadataStore: Metadata
       case batchTimedValuesTry: KvStoreBatchResponse =>
         batchTimedValuesTry.response match {
 
-          case Failure(_)                      => existingServingInfo
-          case Success(value) if value.isEmpty => existingServingInfo
+          case Failure(_)                                       => existingServingInfo
+          case Success(value) if value == null || value.isEmpty => existingServingInfo
 
           case Success(value) if value.iterator.map(_.millis).max <= existingServingInfo.batchEndTsMillis =>
             existingServingInfo