Ensure states with null m2o FK value are still populated by selectinloader

Ensure states with null m2o FK value are still populated by selectinloader

Fixed regression in selectinload loader strategy caused by #4775 (released
in version 1.3.6) where a many-to-one attribute of None would no longer be
populated by the loader.  While this was usually not noticeable due to the
lazyloader populating None upon get, it would lead to a detached instance
error if the object were detached.

Fixes: #4872
Change-Id: I18466841683111643810ebc4331376df38c4767b