fix: remove empty transform struct when generating document URLs

[douglastofoli]

Problem

When generating signed URLs for documents, the transform field was being sent as an empty struct (%Transform{}). This caused the Supabase backend to attempt applying image transformations to documents. The backend expects transform to be nil when not used, especially for non-image files like documents.

Solution

• Updated the code to set transform to nil when not provided or explicitly set to nil in the options.
• Modified the request body to include the transform field only when it is not nil

Rationale

This fix ensures that the Supabase backend does not attempt to apply image transformations to documents, avoiding unnecessary errors. By conditionally including the transform field, the request is cleaner and more accurate. The solution is simple, maintains backward compatibility, and works seamlessly for both document and image URL generation.