Fix cleardb test

Although I find `assert url['PORT'] is None` more pythonic, PORT is set
in the main script via `'PORT': url.port or '',` so, we are obliged to
use checking for '' in our tests.

test_dj_database_url.py

@@ -54,7 +54,7 @@ class DatabaseTestSuite(unittest.TestCase):
         assert url['HOST'] == 'us-cdbr-east.cleardb.com'
         assert url['USER'] == 'bea6eb025ca0d8'
         assert url['PASSWORD'] == '69772142'
-        assert url['PORT'] is None
+        assert url['PORT'] is ''
 
     def test_database_url(self):
         del os.environ['DATABASE_URL']