fix(storage3): Handle None for non-existent signed URLs

[NafeesMadni]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

The create_signed_urls function raises a pydantic_core._pydantic_core.ValidationError when attempting to generate signed URLs for files that do not exist in a Supabase storage bucket. This occurs because the Supabase API returns null for the signedURL field for non-existent files, but the SDK's SignedUrlsJsonItem Pydantic model expects a string.

Relevant issue: #1455

What is the new behavior?

This PR updates the SignedUrlsJsonItem Pydantic model to correctly mark the signedURL field as Optional[str]. Additionally, the create_signed_urls methods in both the asynchronous and synchronous file API implementations have been modified to gracefully handle None values for signedURL from the API response.

Now, if a requested file does not exist, the corresponding entry in the CreateSignedUrlResponse list will correctly reflect None for signedURL and signedUrl, without causing a validation error.

Additional context

This fix enhances the robustness of the storage client, preventing crashes when querying for signed URLs of potentially non-existent files. Users can now safely request signed URLs for a list of files, even if some are missing, without the application crashing due to Pydantic validation failures.

[o-santi]

This PR also needs to fix the tests and some annotations in order to keep make mypy from not throwing an error.

I'll close it in favor of #1461, where I've applied a very similar fix to it, while keeping the types correct.

[NafeesMadni]

@o-santi Thank you for the feedback! I'm glad the logic was on the right track. I really appreciate you pointing out the mypy requirements. I’ll take a look at #1461 to see how you handled the annotations so I can keep that in mind for future PRs. Happy to see the fix getting merged!