Drop empty _source excludes from top_hits test fixtures

OpenSearch 3.7's FetchSourceContext no longer emits "excludes":[] when
empty. Update the two top_hits assertions in MetricAggregationBuilderTest
to match the new serialization.

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/script/aggregation/dsl/MetricAggregationBuilderTest.java

@@ -410,8 +410,7 @@ void should_build_top_hits_aggregation() {
                 + "      \"seq_no_primary_term\" : false,%n"
                 + "      \"explain\" : false,%n"
                 + "      \"_source\" : {%n"
-                + "        \"includes\" : [ \"name\" ],%n"
-                + "        \"excludes\" : [ ]%n"
+                + "        \"includes\" : [ \"name\" ]%n"
                 + "      }%n"
                 + "    }%n"
                 + "  }%n"
@@ -450,8 +449,7 @@ void should_build_filtered_top_hits_aggregation() {
                 + "          \"seq_no_primary_term\" : false,%n"
                 + "          \"explain\" : false,%n"
                 + "          \"_source\" : {%n"
-                + "            \"includes\" : [ \"name\" ],%n"
-                + "            \"excludes\" : [ ]%n"
+                + "            \"includes\" : [ \"name\" ]%n"
                 + "          }%n"
                 + "        }%n"
                 + "      }%n"