The verb wachten is always combined with the preposition op when meaning to wait for ...(someone of something)
Wachten voor... means to wait in front of/before
Eg: Ik wacht op je voor het station > I'm waiting for you in front of the train station
It sounds like ik wacht op niemand. That would still be correct, right? I wait for noone?
Should be accepted, though generally when you say that sentence on it's own you are probably "actively" waiting for someone at that moment.