field_group_remove_empty_display_groups() should never replace an entity render array with NULL

The module adds field_group_build_entity_groups() as a preprocess function to entity render arrays in the hook_theme_registry_alter() implementation. Thus, the field_group's hook_field_group_build_pre_render_alter() implementation can receive an entity render array as $element and it is passed directly to field_group_remove_empty_display_groups(), which in some edge cases can replace the received render array with a NULL.

Having NULL instead of an entity render array causes the whole entity to not be rendered, since the next preprocess hook will get NULL instead of a render array and this makes the whole page crash with a fatal error.

There would be a couple solutions to this, but, I think the most elegant one is to pass in the $group_name to nested calls, and only use the $empty_group flag to empty the $elements array when there is a $group_name present.