[fix] Broken groups management by #4308

https://github.com/joomla/joomla-cms/pull/4308 added an addittional else statement to the user group model that avoids you to edit groups. 

This should be an high priority PR. 

### Steps to reproduce the issue

Go to user groups management and try to edit a group. You get an error. 

### How to test

Check that after applying the patch you can create / edit user groups correctly