Yes! If your guest opts not to bring a plus one, they will be able to RSVP on behalf of that guest when submitting their own RSVP. They can choose "Will Not Attend" for their plus one; if the plus one is set as “Guest Name Unknown” they will not be prompted to add that name when declining the invite. They will still be able to set themselves as attending, and will allow them to select the food options and answer any provided questions.
If they are bringing a guest but they are still not sure who, they will need to add a name to the field, even if it just “Guest Unknown”.