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.
Add HAL+JSON formatter, by @amitaibu, @mateu-aguilo-bosch, @mirzu.
Don't throw exception in list of entities.

If no IDs were requested, we should not throw an exception in case an
entity is un-accessible by the user.