If no owner availability is given, assume avail for full event

If no availability is set for an owner then the current check thinks that the owner is not available at all. This is wrong as no default availability is set AFAIK.

This PR changes the checks s.t. if no availability is given, availability for the full event is assumed.

requested review from @lorax66

added bug label

mentioned in commit 6ad0b9f9

merged