[Test] Fix InternalStatsTests

After recent changes in InternalStats#doXContentBody the corresponding xContent
output of the parsed aggregation needed to be changed in a similar way.

core/src/main/java/org/elasticsearch/search/aggregations/metrics/stats/ParsedStats.java

@@ -109,7 +109,7 @@ public class ParsedStats extends ParsedAggregation implements Stats {
             builder.nullField(Fields.MIN);
             builder.nullField(Fields.MAX);
             builder.nullField(Fields.AVG);
-            builder.nullField(Fields.SUM);
+            builder.field(Fields.SUM, 0.0d);
         }
         otherStatsToXContent(builder, params);
         return builder;