You can’t use that slot to prepare any spells. You still “have” that slot, you just can’t use it for anything. If some feat or whatever allowed you to do something else with your domain slot, or let you prepare other spells from that slot, or just gave you another domain so you could prepare domain spells, then you could use it. But with nothing to prepare there, you cannot use it for anything.
That includes attempting to use the slot to spontaneously cast cure instead. Domain slots cannot be used for that even when you have them.
Generally speaking, it is best for clerics (at least, those who intend to stay cleric instead of just dipping the class for one level, which is a popular choice) to not trade away all their domains for domain feats for exactly this reason.
Houseruling the situation to allow for using those slots to spontaneously cast cure (or inflict) seems fair to me, but I would still recommend against doing that. Cure (and inflict) spells are awful, and while it’s nice to always have them available in case of emergency, giving up the opportunities you could have from your domain spells is bad, and getting cure (or inflict) spells in return doesn’t change that.