Fix loading streaming Bedrock response with tool usage with empty argument

[pawel-dabro]

Why are these changes needed?

When using streaming with Bedrock client, the tool use response may not contain partialJson increments for tool argument if that argument is empty. In this case an empty string was used as input instead of the input provided in the starting chunk of the tool use block (empty json object). This led to an error during subsequent attempted tool invocation, when an empty string was being parsed as a json.
Note that added Bedrock test cases only run if AWS credentials are present in the environment variables.

Related issue number

Closes #6661

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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.

[pawel-dabro]

@microsoft-github-policy-service agree

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.99%. Comparing base (82df9dd) to head (8bc3834).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../autogen_ext/models/anthropic/_anthropic_client.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6979      +/-   ##
==========================================
- Coverage   81.01%   80.99%   -0.02%     
==========================================
  Files         237      237              
  Lines       18182    18186       +4     
==========================================
  Hits        14730    14730              
- Misses       3452     3456       +4     

Flag	Coverage Δ
unittests	80.99% <0.00%> (-0.02%)	⬇️

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