Describe the bug
Sometimes when using the openrouter API endpoint with the OpenAIServerModel you will get the following error:

Error in generating model output:
'NoneType' object has no attribute 'prompt_tokens'

This is because openrouter doesn't guarantee the usage key.

Here is the openrouter API page: https://openrouter.ai/docs/api-reference/overview#completionsresponse-format
Here is the openapi page: https://platform.openai.com/docs/api-reference/chat/object

Code to reproduce the error
Use the openrouter API endpoint with the OpenAIServerModel, occasionally this error will materialize.

Error logs (if any)

Error in generating model output:
'NoneType' object has no attribute 'prompt_tokens'

Expected behavior
Defaults to 0 when response.usage is None.

Packages version:
smolagents 1.17.0

Additional context
Add any other context about the problem here.