Prevent empty field group to be displayed

Problem/Motivation

Original GH issue:
Original GH PR: https://github.com/nuvoleweb/ui_patterns/pull/337
Follow (github) : https://github.com/nuvoleweb/ui_patterns/pull/336
Follow (drupal.org) : #3314998: Allow to use field_group label in patterns

We don't want empty field groups to be displayed if they only contain a label or another empty field group unless the "Display element also when empty" option is enabled.