Hashed Storage Migration: Handle failed attachment migrations with existing target path

What does this MR do?

There are few cases were the migration could fail due to a racing condition, in which the uploader creates temporary folders on the new hashed directory, before we migrate the files.

This is a workaround to that situation, which allow us to ignore the temporary folder structure when it is completely empty. We will move the folder to a backup one in order to cleanup the space, but will not remove in order to prevent data-loss.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #32367 (closed)