Fix empty array when retrieving stats data (feast-dev#930)

Author: terryyylim

core/src/main/java/feast/core/service/StatsService.java

@@ -129,6 +129,9 @@ public GetFeatureStatisticsResponse getFeatureStatistics(GetFeatureStatisticsReq
         List<FeatureNameStatistics> featureNameStatistics =
             getFeatureNameStatisticsByDate(
                 statisticsRetriever, featureSet, features, timestamp, request.getForceRefresh());
+        if (featureNameStatistics.size() != 0) {
+          featureNameStatisticsList.add(featureNameStatistics);
+        }
         featureNameStatisticsList.add(featureNameStatistics);
         timestamp += 86400; // advance by a day
       }
@@ -147,14 +150,16 @@ public GetFeatureStatisticsResponse getFeatureStatistics(GetFeatureStatisticsReq
         List<FeatureNameStatistics> featureNameStatistics =
             getFeatureNameStatisticsByDataset(
                 statisticsRetriever, featureSet, features, datasetId, request.getForceRefresh());
-        featureNameStatisticsList.add(featureNameStatistics);
-        if (featureNameStatisticsList.size() == 0) {
-          throw new RetrievalException(
-              String.format(
-                  "Unable to find any data over provided data sets %s",
-                  request.getIngestionIdsList()));
+        if (featureNameStatistics.size() != 0) {
+          featureNameStatisticsList.add(featureNameStatistics);
         }
       }
+      if (featureNameStatisticsList.size() == 0) {
+        throw new RetrievalException(
+            String.format(
+                "Unable to find any data over provided data sets %s",
+                request.getIngestionIdsList()));
+      }
     }
 
     // Merge statistics values across days/datasets