Handeling of empty hostgroups in maintenance is not consistent.
Currently, empty groups do not show up as selectable in the maintenance configuration. However:

1) we create a maintenance on a host group with hosts
2) we remove the hosts from the hostgroup
3) opening the maintenance config will not show any groups assigned now.
4) Pushing 'Change' will throw the error: "At least one host or group should be selected."
5) If we cancel changes and add a host to the group, the group will now show up again in the maintenance.

I think the best way to solve this is to actually allow empty groups in maintenance as it is not uncommon to dynamically add and remove hosts from groups.