Force orb only makes the secondary attack if it hits. So logically you can't target "nothing" with it and just have it explode, it needs to smash on something. you could target the floor to about as much effect, or a chair or a flag pole. I'd probably draw the line somewhere before dandelions and floating dust, though.
You couldn't, however, just have it explode mid air to hit multiple flying targets. You can't target air with it, hitting air is a miss. (note: this is only for force orb and similar ranged attacks, any burst can be centered on open air just fine)