[1.8.x] Fixed #24307: Avoided redundant column nullability modifications on Oracle

Thanks Joris Benschop for the report, and Tim Graham for the tests.

Backport of ceadc94f09 from master

django/db/backends/base/schema.py

@@ -593,7 +593,11 @@ class BaseDatabaseSchemaEditor(object):
                 ))
         # Nullability change?
         if old_field.null != new_field.null:
-            if new_field.null:
+            if (self.connection.features.interprets_empty_strings_as_nulls and
+                    new_field.get_internal_type() in ("CharField", "TextField")):
+                # The field is nullable in the database anyway, leave it alone
+                pass
+            elif new_field.null:
                 null_actions.append((
                     self.sql_alter_column_null % {
                         "column": self.quote_name(new_field.column),