add tolerance for empty choices

[MohMaz]

Why are these changes needed?

I encountered this issue only in AzureOpenAIChatCompletionClient, adding some tolerance for empty choices ! Not a super clean fix ... just works to unblock updating streaming samples using AzureOpenAIChatCompletionClient

Related issue number

closes #4213

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.

[jspv]

Can we extend the fix to be an argument or property to the AzureOpenAIChatCompletionClient object rather than an argument to create() and create_stream() and have create() and create_stream() check the object? This way Agents who call create() and create_stream() don't need to also be changed to implement this fix, the parameter can be set when instantiating the model client and then passed to the agent via model_client= when the Agent is instantiated.