Fix numpy warning when stats don't exist (#3251)

ml-agents/mlagents/trainers/stats.py

     std: float
     num: int
 
+    @staticmethod
+    def empty() -> "StatsSummary":
+        return StatsSummary(0.0, 0.0, 0)
+
 
 class StatsWriter(abc.ABC):
     """
         :param key: The type of statistic, e.g. Environment/Reward.
         :returns: A StatsSummary NamedTuple containing (mean, std, count).
         """
-        return StatsSummary(
-            mean=np.mean(StatsReporter.stats_dict[self.category][key]),
-            std=np.std(StatsReporter.stats_dict[self.category][key]),
-            num=len(StatsReporter.stats_dict[self.category][key]),
-        )
+        if len(StatsReporter.stats_dict[self.category][key]) > 0:
+            return StatsSummary(
+                mean=np.mean(StatsReporter.stats_dict[self.category][key]),
+                std=np.std(StatsReporter.stats_dict[self.category][key]),
+                num=len(StatsReporter.stats_dict[self.category][key]),
+            )
+        return StatsSummary.empty()