This fix allows the listbox items to accept "null" values,
which can represent a "no value" or "default" state.

feat(GlCollapsibleListbox): Allow listbox options to have a null value

See merge request !5047



Merged-by: Thomas Hutterer <thutterer@gitlab.com>
Approved-by: Denys Mishunov <dmishunov@gitlab.com>
Approved-by: Thomas Hutterer <thutterer@gitlab.com>
Reviewed-by: Miguel Rincon <mrincon@gitlab.com>
Reviewed-by: Denys Mishunov <dmishunov@gitlab.com>
Co-authored-by: Miguel Rincon <mrincon@gitlab.com>

# [111.10.0](gitlab-org/gitlab-ui@v111.9.1...v111.10.0) (2025-03-28)

### Features

* **GlCollapsibleListbox:** Allow listbox options to have a null value ([6e1c5f6a](gitlab-org/gitlab-ui@6e1c5f6a))