You can activate ZERO-MAX even if there are no monsters on the field.
You can only activate "ZERO-MAX" by having it Set on the field and flipping it face-up, while you have zero cards in your hand. If you only have one copy of "ZERO-MAX" in your hand, then you cannot activate that "ZERO-MAX" from your hand.
The effect of "ZERO-MAX" targets one "Infernity" monster in your Graveyard.
The effect of "ZERO-MAX" which Special Summons an "Infernity" monster and the effect which destroys all face-up monsters are not treated as resolving at the same time. Even if there are no monsters on the field, then it is treated as finishing resolving with the destruction of monsters. Thus, even if "ZERO-MAX" does not destroy monsters, then you cannot activate cards like "Torrential Tribute" because you miss the timing.