If new related entities do not exist, use an empty set. NEEDS TEST to show failure case.
If new related entities do not exist, use an empty set. NEEDS TEST to show failure case.

Example:

  File "/Users/gldnspud/env/atfgx/src/schevo/schevo/database2.py", line 940, in _update_entity
    - new_related_entities_by_id[referrer_field_id]
KeyError: 1411265725

Signed-off-by: Matthew R. Scott <878b2bb7d7b44067d87275810e479f4abd7737ae@gmail.com>

Schevo/schevo,Schevo/schevo