deprecate using None for code_execution_config

[jackgerrits]

Why are these changes needed?

As a user I would expect providing a value of None for code_execution_config would disable the feature. It is instead used as a sentinel for the "default" case. This change is the first step in rectifying this confusion.

Related issue number

N/A

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 (5eaf712) 33.08% compared to head (2bb3c31) 53.59%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1506       +/-   ##
===========================================
+ Coverage   33.08%   53.59%   +20.51%     
===========================================
  Files          42       42               
  Lines        5048     5051        +3     
  Branches     1156     1224       +68     
===========================================
+ Hits         1670     2707     +1037     
+ Misses       3250     2112     -1138     
- Partials      128      232      +104     

Flag	Coverage Δ
unittests	53.53% <100.00%> (+20.49%)	⬆️

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[ekzhu]

LGTM

[afourney]

LGTM