Read size remotely if value is null (Fix #11743)

If a file has not yet been updated by the server,
then the `size` field could still be null. In that
case, we ask the service to tell us the size.
Passing test in test_rawfilestore.py (See #11743)

Shows that loading the size from the service is possible
even when the size field itself is None/null.
Fix non-handling of ServerError
Merge pull request #1829 from joshmoore/11743-null-size

Read size remotely if value is null (Fix #11743)