Make list empty when making empty summary

ml-agents/mlagents/trainers/stats.py

 
     @staticmethod
     def empty() -> "StatsSummary":
-        return StatsSummary([0.0], StatsAggregationMethod.AVERAGE)
+        return StatsSummary([], StatsAggregationMethod.AVERAGE)
 
     @property
     def aggregated_value(self):