audit: return if no github license were found

[dawidd6]

Fixes audit problems when GitHub API returns nil license, like this:
Homebrew/homebrew-core#59865 (comment)

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[dtrodrigues]

The intention in #8366 was to only audit if a GitHub license was present, not sure how I missed this.