Possibly confusing ROV status when VRP cache is empty

[DRiKE]

Calling rpki.check_rov from roto currently marks a route as NotFound when the VRP cache is completely empty. While this might be 'technically correct' and one could argue this is by the 'fail open' design of ROV, perhaps we should introduce another variant next to NotChecked and NotFound that hints at the fact that an ROV attempt was done but no RPKI data came in via RTR (yet).