rest, indexer: allow the {user,project}_id to be empty

Now that we track who created the resources, we can allow these fields
to be empty as they're only there for informational purpose for the API
consumer.

Change-Id: Ib0658576a56a553f6856adccfc74c6f929d2d5d0
Merge "rest, indexer: allow the {user,project}_id to be empty"