Fix `generate_init_message` for Multimodal Messages

[WaelKarkoub]

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Why are these changes needed?

Related issue number

Closes #1924

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.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 56.78%. Comparing base (061a857) to head (0ce30ad). Report is 1424 commits behind head on main.

Files with missing lines	Patch %	Lines
autogen/agentchat/conversable_agent.py	76.00%	3 Missing and 3 partials :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2124       +/-   ##
===========================================
+ Coverage   37.54%   56.78%   +19.23%     
===========================================
  Files          77       77               
  Lines        7711     7719        +8     
  Branches     1655     1793      +138     
===========================================
+ Hits         2895     4383     +1488     
+ Misses       4573     2971     -1602     
- Partials      243      365      +122     

Flag	Coverage Δ
unittest	14.21% <12.00%> (?)
unittests	55.75% <76.00%> (+18.22%)	:arrow_up:

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