Product variant image fallback logic has been moved out of a shared image component and kept where it is actually used. This reduces unnecessary complexity and improves test coverage by checking image content, not just image size.

part of task-1949729

Several buttons in the Documents app could trigger an error when used after a recent change. This fix restores those button actions so users can save and continue their work without encountering a crash.

The changes in odoo/odoo@1ced3bf
highlighted an error in several buttons in documents' views and a
traceback was generated when those buttons were used.

This commit fixes this issue.