Fix: check response usage is not None

[olgavrou]

Why are these changes needed?

usage in response is Optional but we never check if it is there, so this fixes that

Related issue number

resolves #1579

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.