There are actually two questions in the OP, so I'll answer them both.
Yes, the power can be used in a place without valid targets.
An Area Attack doesn't aim at a target. It aim at a square (within line of effect, as usual), and usually targets creatures within the area created around the target square. If there is no valid targets in the area, usually this means you throw your magic missile at the darkness and nothing happened. However...
Stinking Cloud creates the zone even if there is no targets in the area
Pretty much every power in 4e have three lines that describes what happens when you use a power. The Hit line describes what happens when, well, your attack roll hits, the Miss line describes what happens when the attack roll miss, and the Effect line describes what happens when you use the power, regardless of its attack roll (or if it doesn't have an attack roll).
The creation of the zone in the power Stinking Cloud is an Effect, and thus, always happens in the designated area regardless of the result (or presence) of the attack roll.