Empty value should not create a new entity from a reference field.
Only check filter access on write contexts.
Merge pull request #258 from HelenaEksler/null-reference

Empty value should not create a new entity from a reference field.

Sometimes a client may send a NULL value, but it might be casted to an empty string. We now make sure with are returning a NULL value in that case.
Merge branch '7.x-1.x' into php-apache-server