Tests: fix MockRequestsBackend.get_api_call_arg edge cases

get_api_call_arg had incorrectly returned None if a kwarg was passed
to the mocked function with a False-y value (e.g., [] or {})

get_api_call_json had only considered data param, ignoring json param
requests added a while back

tests/test_mailgun_backend.py

@@ -108,7 +108,7 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         self.assertEqual(data['html'], html_content)
         # Don't accidentally send the html part as an attachment:
         files = self.get_api_call_files(required=False)
-        self.assertIsNone(files)
+        self.assertFalse(files)
 
     def test_html_only_message(self):
         html_content = '<p>This is an <strong>important</strong> message.</p>'