group property for options in select are not displayed anymore

[sebdeleze]

Describe the issue
If a group is specified in the JSON schema, the options have to be grouped by this group when rendering the select box. It was the case, but a recent PR broke this behavior.

To Reproduce

1. Go to the editor in sonardev.test.rero.ch.
2. In the language select box, the first four options must be separated from others.

Expected behavior
A separator representing the group have to be displayed after the fourth options (preferred languages).

Screenshots