Return old_path when new_path is nil

What does this MR do and why?

As part of a DB space saving approach, we are exploring NOT saving new_path when old_path == new_path. This change supports that, but assuming that when new_path.nil?, the path has not changed from old_path, and thus old_path is the correct value to send. This allows us to more seamlessly drop in a nullification of new_path, rather than having to change our code-wide expectations of what a nil new_path value means.

Related to #557745 (closed) and &11272 (comment 2648942537)