fix assistant creating without file

[IANTHEREAL]

Why are these changes needed?

Creating an assistant with file ids, otherwise the retrieval function cannot be used.

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.

[sonichi]

Thanks. Adding a test will be nice, though not mandatory for this PR.

[gagb]

Btw there is an issue for adding a notebook example as well (tho not a substitute for the test) #641

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b0a6d72) 28.85% compared to head (ffbdd47) 48.30%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #689       +/-   ##
===========================================
+ Coverage   28.85%   48.30%   +19.45%     
===========================================
  Files          27       27               
  Lines        3393     3393               
  Branches      764      806       +42     
===========================================
+ Hits          979     1639      +660     
+ Misses       2342     1577      -765     
- Partials       72      177      +105     

Flag	Coverage Δ
unittests	48.12% <ø> (+19.33%)	⬆️

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[IANTHEREAL]

@sonichi PTAL again, a simple assistant file checking unit test is added