Do not create empty files with SQLite ':memory:' with `django_testdir`

I have noticed that there are `test_:memory:_db_test` files in the testdirs created from the `django_testdir` fixture.

 - Keep ':memory:' for DB_NAME and TEST_DB_NAME with SQLite in-memory DB.
 - Skip `create_empty_production_database` with DB_NAME == ':memory:'.