Modify validity check from r16678 slightly to work with Oracle.

We now skip this particular check on Oracle backends. Change is based on
a backend feature check, so it also works with other backends that may
treat NULL as equal to an empty string.

Fixes #16694 (with luck).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16681 bcc190cf-cafb-0310-a4f2-bffc1f526a37