Prevent Oracle from changing field.null to True

Fixed #17957 -- when using Oracle and character fields, the fields
were set null = True to ease the handling of empty strings. This
caused problems when using multiple databases from different vendors,
or when the character field happened to be also a primary key.

The handling was changed so that NOT NULL is not emitted on Oracle
even if field.null = False, and field.null is not touched otherwise.

Thanks to bhuztez for the report, ramiro for triaging & comments,
ikelly for the patch and alex for reviewing.

tests/regressiontests/queries/tests.py

@@ -1930,3 +1930,25 @@ class NullInExcludeTest(TestCase):
         self.assertQuerysetEqual(
             NullableName.objects.exclude(name__in=[None]),
             ['i1'], attrgetter('name'))
+
+class EmptyStringsAsNullTest(TestCase):
+    """
+    Test that filtering on non-null character fields works as expected.
+    The reason for these tests is that Oracle treats '' as NULL, and this
+    can cause problems in query construction. Refs #17957.
+    """
+
+    def setUp(self):
+        self.nc = NamedCategory.objects.create(name='')
+
+    def test_direct_exclude(self):
+        self.assertQuerysetEqual(
+            NamedCategory.objects.exclude(name__in=['nonexisting']),
+            [self.nc.pk], attrgetter('pk')
+        )
+
+    def test_joined_exclude(self):
+        self.assertQuerysetEqual(
+            DumbCategory.objects.exclude(namedcategory__name__in=['nonexisting']),
+            [self.nc.pk], attrgetter('pk')
+        )