Is there a criteria for a decision diamond that can read “if the contact has no owner” then…?
There isn’t an option for ’no owner’, but you could use the negative and say “does not equal” and then choose the list of all active users.
What are you trying to accomplish? Are you trying to assign an owner if they don’t have one?
If that is the case, you can use the ‘assign an owner’ process inside a sequence. In there they have an option for “do not assign an owner if already assigned” — which essentially says “If they don’t have an owner, then assign one”.