To my mind, you can target any power at an empty space. However, you only get the effects it says you get. You can aim a Sign of Vulnerability at an empty square, but if there's no invisible enemy there, it does nothing (because there's nothing there to take the half-onna-miss). You can target a force orb at an empty space, but it won't do anything unless A) there is actually an invisible creature there, and B) you hit said creature.
I have really no problem with Force Orbing the floor. All you're doing if you do that is spending an encounter power to get a very slightly more powerful version of Scorching Burst. In my campaign, if you target an object, you're targetting one square of the object and striking all creatures adjacent to that square, not adjacent to the whole object.
I have really no problem with Force Orbing the floor. All you're doing if you do that is spending an encounter power to get a very slightly more powerful version of Scorching Burst. In my campaign, if you target an object, you're targetting one square of the object and striking all creatures adjacent to that square, not adjacent to the whole object.