Fixed #7833 -- Improved UserCreationForm password validation

Make UserCreationForm password validation similar to
SetPasswordForm and AdminPasswordChangeForm, so as the match
check is only done when both passwords are supplied.
Thanks Mitar for the suggestion.

django/contrib/auth/tests/forms.py

@@ -65,6 +65,7 @@ class UserCreationFormTest(TestCase):
         form = UserCreationForm(data)
         self.assertFalse(form.is_valid())
         self.assertEqual(form['password1'].errors, required_error)
+        self.assertEqual(form['password2'].errors, [])
 
     def test_success(self):
         # The success case.