redundancy code clean for agentchat

[zhanluxianshen]

Why are these changes needed?

1 redundancy code
2 first check error, then do logic.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.27%. Comparing base (3932126) to head (7b830de).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...chat/teams/_group_chat/_base_group_chat_manager.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6190   +/-   ##
=======================================
  Coverage   77.27%   77.27%           
=======================================
  Files         200      200           
  Lines       14237    14237           
=======================================
  Hits        11002    11002           
  Misses       3235     3235           

Flag	Coverage Δ
unittests	77.27% <80.00%> (ø)

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[zhanluxianshen]

thanks.