Fix function call responses from Azure throwing errors

[karlerss-teamdash]

The responses from Azure for gpt-35-turbo-16k don't seem to have the content attribute (with OpenAI it's set to null).

This PR fixes the ErrorException: Undefined array key "content" error.

[ukkok]

Just faced same issue and fixed with exactly same solution. Can confirm that this works.

From Azure documentation, content should not be returned by default:

In some cases, the model may generate both content and a function_call.

https://learn.microsoft.com/en-us/azure/ai-services/openai/how-to/function-calling#using-function-in-the-chat-completions-api

[gehrisandro]

@karlerss-teamdash Many thanks for the PR!