Update time_series_query.go

fix alert no data when elasticsearch group by terms size is set to no limit
(cherry picked from commit 3b9ab6e204)

pkg/tsdb/elasticsearch/time_series_query.go

@@ -171,6 +171,10 @@ func addTermsAgg(aggBuilder es.AggBuilder, bucketAgg *BucketAgg, metrics []*Metr
 		} else {
 			a.Size = 500
 		}
+		if a.Size == 0 {
+		    a.Size = 500
+		}
+		
 		if minDocCount, err := bucketAgg.Settings.Get("min_doc_count").Int(); err == nil {
 			a.MinDocCount = &minDocCount
 		}