If you control no creatures, you do nothing as Primal Empathy's ability resolves. If there are no creatures on the battlefield at all, you don't draw a card.

If the greatest power among creatures on the battlefield is shared by a creature you control and a creature you don't control, you draw a card.

If you don't control a creature with the greatest power, you choose which creature gets a +1/+1 counter while the ability is resolving. Players can't take actions in between the time you make this choice and the time that creature has a +1/+1 counter.

You determine whether you draw a card or put a +1/+1 counter only as Primal Empathy's triggered ability begins resolving. Players may respond to the ability to try to change which happens.