return None instead of tuple in _generate_oai_reply_from_client

[sonichi]

Why are these changes needed?

Fix a bug in _generate_oai_reply_from_client
which causes test failures when the extracted response is None.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5c58be6) 37.79% compared to head (75a6946) 68.12%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1644       +/-   ##
===========================================
+ Coverage   37.79%   68.12%   +30.32%     
===========================================
  Files          50       50               
  Lines        5704     5704               
  Branches     1295     1404      +109     
===========================================
+ Hits         2156     3886     +1730     
+ Misses       3377     1464     -1913     
- Partials      171      354      +183     

Flag	Coverage Δ
unittests	68.11% <100.00%> (+30.31%)	⬆️

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