Revert "Don't return a Country object for a null or empty string"

This reverts commit dc57acf8f1125b7aa15e59a560ca2b92abba8381.

Conflicts:
	django_countries/fields.py
	django_countries/tests/test_fields.py
Revert descriptor to always return a Country object again

While the change for #47 was reverted in the previous commit, this
should also make the requested behaviour work as well (country=Null
actually commits NULL).

Fixes #82