Fixed credential test

Author: barshaul

tests/test_credentials.py

@@ -234,7 +234,7 @@ def test_user_pass_credential_provider_acl_user_and_pass(self, r, request):
     def test_user_pass_provider_only_password(self, r, request):
         password = "password"
         provider = UsernamePasswordCredentialProvider(password=password)
-        assert provider.username is None
+        assert provider.username is ""
         assert provider.password == password
         assert provider.get_credentials() == (password,)