ui: vm selector: gracefully handle empty IDs in setValue function

ui: vm selector: gracefully handle empty IDs in setValue function

An empty string is passed by the backup job window when using
selection mode 'all', would be converted to [""] and wrongly add an
entry with VMID 0 because the item "" could not be found in the store.

Reported in the community forum:
https://forum.proxmox.com/threads/130164/

Fixes: 7a5ca76a ("fix #4239: ui: show selected but non-existing vmids in backup edit")
Suggested-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>