Don't call NotifyDone in Agent.OnDisable if Academy is shut down (#4489)

com.unity.ml-agents/CHANGELOG.md

 #### com.unity.ml-agents (C#)
 - Previously, `com.unity.ml-agents` was not declaring built-in packages as
 dependencies in its package.json. The relevant dependencies are now listed. (#4384)
+- Agents no longer try to send observations when they become disabled if the
+Academy has been shut down. (#4489)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Fixed the sample code in the custom SideChannel example. (#4466)
 - A bug in the observation normalizer that would cause rewards to decrease

com.unity.ml-agents/Runtime/Agent.cs

                 Academy.Instance.DecideAction -= DecideAction;
                 Academy.Instance.AgentAct -= AgentStep;
                 Academy.Instance.AgentForceReset -= _AgentReset;
+                NotifyAgentDone(DoneReason.Disabled);
-            NotifyAgentDone(DoneReason.Disabled);
             m_Brain?.Dispose();
             m_Initialized = false;
         }