Fixed #21746 - Fixed test_get_reverse_on_unsaved_object test.

Test will no longer attempt to insert multiple NULL rows in to a unique
field if the database doesn't support it.

tests/one_to_one_regress/tests.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from django.db import connection
 from django.test import TestCase
 
 from .models import (Bar, Favorites, HiddenPointer, Place, Restaurant, Target,
@@ -226,6 +227,9 @@ class OneToOneRegressionTests(TestCase):
             with self.assertRaises(UndergroundBar.DoesNotExist):
                 p.undergroundbar
 
+        # Several instances of the origin are only possible if database allows
+        # inserting multiple NULL rows for a unique constraint
+        if connection.features.ignores_nulls_in_unique_constraints:
             UndergroundBar.objects.create()
 
             # When there are several instances of the origin