SAK-22519 Correctly detect a maintainer when uploading file to dropbox.

When a maintainer uploaded a new file to a dropbox and checked notification the id was set without the path which meant that the dropboxId couldn't be found. This makes it fallback to the containingCollectionId if the lookup failed.

git-svn-id: https://source.sakaiproject.org/svn/content/trunk@120126 66ffb92e-73f9-0310-93c1-f5514f145a0a