When a group has a falsy value (e.g. if grouped by an integer field having a "0" group), make sure to display its proper name instead of "Undefined".
Off course, boolean fields are still showing "false" name if it is the case.
This issue was introduced with 3dd1c0f
closes odoo/odoo#76493
X-original-commit: 4e213f0b93f59cf377271b18eae9765ff9cfbe4f
Related: odoo/enterprise#20847
Signed-off-by: Mathieu Duckerts-Antoine <Polymorphe57@users.noreply.github.com>