[1.5.x] Fixed empty strings + to_field regression on Oracle

Querying the reverse side of nullable to_field relation, where both
sides can contain null values resulted in incorrect results. The reason
was not detecting '' as NULL.

Refs #17541, backpatch of 09fcb70c80.

tests/regressiontests/many_to_one_regress/tests.py

@@ -126,7 +126,7 @@ class ManyToOneRegressionTests(TestCase):
         # Now the model is saved, so we will need to execute an query.
         with self.assertNumQueries(1):
             self.assertEqual(th.child_set.count(), 0)
-    
+
     def test_related_null_to_field(self):
         c1 = Car.objects.create()
         c2 = Car.objects.create()