Fix the situation where output_tokens/input_tokens may be None in response.usage

[AAlexDing]

Summary

The usage information returned by some 3rd-party API service providers may be None, which will affect the calculation of the get_price function and lead to errors unsupported operand type(s) for *: 'NoneType' and 'decimal.Decimal' during multiplication.

The usage information returned by some 3rd-party API service providers :
"usage":{"input_tokens":0,"output_tokens":None}

Screenshots

Before:

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

Please fix the lint errors in CI actions.