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

• [x] 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.