Remove default trigger value for register_nested_chats

[ekzhu]

Why are these changes needed?

The trigger argument must not have default value. Because for every case it requires user to set some specific agent for the trigger. If it were set to any agent, then any nested chat is going to trigger a recursive nested chat.

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 ✅

Project coverage is 54.56%. Comparing base (0a49f2a) to head (2bafdb2).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1833       +/-   ##
===========================================
+ Coverage   35.69%   54.56%   +18.86%     
===========================================
  Files          63       63               
  Lines        6625     6625               
  Branches     1454     1579      +125     
===========================================
+ Hits         2365     3615     +1250     
+ Misses       4067     2713     -1354     
- Partials      193      297      +104     

Flag	Coverage Δ
unittests	54.52% <ø> (+18.82%)	⬆️

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[qingyun-wu]

LGTM. Thanks!