django-ldap-auth expects the "unset/empty" state of certain LDAP DN
settings (such as AUTH_LDAP_REQUIRE_GROUP and
AUTH_LDAP_USER_DN_TEMPLATE) to be NULL/None (not an empty string).
Resolves#4678
This reverts commit af8e6f939ff2d3ecf4658e82777dcd35c0d288a8.
This test seems to leak django.conf.settings between tests (causing the
suite to fail periodically). Rolling this back until I can figure out
why.