[bug] Check for non-null event before invoking. Fixes MLA-217. (#2782)

UnitySDK/Assets/ML-Agents/Scripts/Academy.cs

             Time.maximumDeltaTime = m_OriginalMaximumDeltaTime;
 
             // Signal to listeners that the academy is being destroyed now
-            DestroyAction();
+            DestroyAction?.Invoke();
 
             foreach (var mr in m_ModelRunners)
             {