Equate '' to None for foreign key references when PUTing to a job template
Equate '' to None for foreign key references when PUTing to a job template

The UI will sometimes send an empty string instead of null, we already
handle this in our serializer and treat it as null, so this change
brings our access system in line with the expectation that those two
things should be equal.

 Possibly related to #3077

snahelou/awx,wwitzel3/awx,snahelou/awx,wwitzel3/awx,snahelou/awx,snahelou/awx,wwitzel3/awx,wwitzel3/awx