Return null for aggregations factory even if empty aggs is present in search source (#8206)

* Return null for aggregations factory even if empty aggs is present in search source

Signed-off-by: Jay Deng <jayd0104@gmail.com>

* Update rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/60_empty.yml

Co-authored-by: Andriy Redko <drreta@gmail.com>
Signed-off-by: Jay Deng <jayd0104@gmail.com>

---------

Signed-off-by: Jay Deng <jayd0104@gmail.com>
Co-authored-by: Andriy Redko <drreta@gmail.com>
(cherry picked from commit 049129c)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/60_empty.yml

@@ -0,0 +1,29 @@
+---
+"Empty aggs Body":
+  - do:
+      index:
+        index: test
+        id: 1
+        body: { "double" : 42 }
+
+  - do:
+      index:
+        index: test
+        id: 2
+        body: { "double" : 100 }
+
+  - do:
+      index:
+        index: test
+        id: 3
+        body: { "double" : 50 }
+
+  - do:
+      indices.refresh: {}
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        body: { "size" : 0, "aggs" : { } }
+
+  - match: { hits.total: 3 }

server/src/main/java/org/opensearch/search/aggregations/AggregatorFactories.java

@@ -235,7 +235,7 @@ private static AggregatorFactories.Builder parseAggregators(XContentParser parse
             }
         }
 
-        return factories;
+        return factories.count() > 0 ? factories : null;
     }
 
     public static final AggregatorFactories EMPTY = new AggregatorFactories(new AggregatorFactory[0]);