"Wasting Powers" for a Reason other than hitting the enemy

Miri's last encounter power is Fire Shroud which is a close burst 3 power. If there are no enemies that Miri can detect within three squares, can she just fire it off anyway? (Hey, there's a chance isn't there that Hayden is evil and in the area, right?)

This comes up as Miri gains some bonus to her at Will Powers if all of her encounter powers have been used.

Is this different than Sonya thinking there might be an invisible hidden enemy that she can't target in the square next to her and using Lion's Roar so that Sonya can heal a nearby ally?

I think you can do a blast and burst whenever you want. You just can't do target specific attacks to use powers at say a tree (unless it's a living tree trying to pummel you to death).