fix: GroupChatManager async run throw exception if no eligible speaker (#4283)

Author: leryor

autogen/agentchat/groupchat.py

@@ -1264,6 +1264,10 @@ async def a_run_chat(
                 else:
                     # admin agent is not found in the participants
                     raise
+            except NoEligibleSpeaker:
+                # No eligible speaker, terminate the conversation
+                break
+
             if reply is None:
                 break
             # The speaker sends the message without requesting a reply