Is there a storage tank up in the attic?

I'd also open all of the hot taps in the house to allow the system to drain -- if air comes in, water won't be "stuck" in the pipes.

Other alternative is to plug the line with a piece of bread. (No joke.) Solder a threaded nipple onto the end. Remove the bread plug. Screw on a valve. Close the valve. Then go about attaching the water heater. No drip, no mess, easy to solder.