Fix UT

Signed-off-by: Lantao Jin <ltjin@amazon.com>

Author: LantaoJin

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/scan/OpenSearchIndexScanTest.java

@@ -130,7 +130,7 @@ void serialize(Integer numberOfIncludes) {
     request.setScrollId("valid-id");
     // make a response, so OpenSearchResponse::isEmpty would return true and unset needClean
     var response = mock(SearchResponse.class);
-    when(response.getAggregations()).thenReturn(mock());
+    when(response.getAggregations()).thenReturn(null);
     var hits = mock(SearchHits.class);
     when(response.getHits()).thenReturn(hits);
     when(response.getScrollId()).thenReturn("valid-id");