fix(backups/RemoteAdapter#deleteVmBackups): dont delete undefined
fix(backups/RemoteAdapter#deleteVmBackups): dont delete undefined

Fixes https://xcp-ng.org/forum/topic/5331/backup-smart-mode-broken/6
Introduced by 7e302fd1c

lmcro/xo-web,lmcro/xo-web,vatesfr/xo-web,lmcro/xo-web,vatesfr/xo-web