[1.7.x] Fixed #22816 -- Corrected Oracle feature flag and fixed introspection test

Backport of 61229040 from master

django/db/backends/__init__.py

@@ -666,6 +666,10 @@ class BaseDatabaseFeatures(object):
     can_introspect_max_length = True
 
     # Can the backend determine reliably if a field is nullable?
+    # Note that this is separate from interprets_empty_strings_as_nulls,
+    # although the latter feature, when true, interferes with correct
+    # setting (and introspection) of CharFields' nullability.
+    # This is True for all core backends.
     can_introspect_null = True
 
     # Confirm support for introspected foreign keys

django/db/backends/oracle/base.py

@@ -111,7 +111,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_tablespaces = True
     supports_sequence_reset = False
     can_introspect_max_length = False
-    can_introspect_null = False
     can_introspect_time_field = False
     atomic_transactions = False
     supports_combined_alters = False

tests/introspection/tests.py

@@ -73,15 +73,14 @@ class IntrospectionTests(TestCase):
             [30, 30, 75]
         )
 
-    # The following test fails on Oracle. Since it forces null=True under the
-    # hood in some cases, its idea about null_ok is different from ours.
     @skipUnlessDBFeature('can_introspect_null')
     def test_get_table_description_nullable(self):
         with connection.cursor() as cursor:
             desc = connection.introspection.get_table_description(cursor, Reporter._meta.db_table)
+        nullable_by_backend = connection.features.interprets_empty_strings_as_nulls
         self.assertEqual(
             [r[6] for r in desc],
-            [False, False, False, False, True, True]
+            [False, nullable_by_backend, nullable_by_backend, nullable_by_backend, True, True]
         )
 
     # Regression test for #9991 - 'real' types in postgres