fietsbob has the right answer, personally would use an allen key to push the pads back in, Hydraulic brakes although an open system are sealed, and although you shouldn't depress the brake lever when the wheel is not inserted, it shouldn't require a bleed.

When you have pushed the pistons out, re-fit the wheel, and pump up the system by squeezing the brake lever until you have the desired feel to the brakes / lever.

For the future, did you bike come with a travel / set-up insert for the brakes, as you can install this when the wheel is removed and then depress the brake lever when ever you want.