Is the garage in question attached to the house ... if so, you will get some heat coming from the house into the garage, and there is certainly a time lag between the outside temperature and the garage temperature, but the temperature in the garage will be closer to the average daily temperature of the outside than to the temperature of the house.
My garage is pretty much contained inside my house, with only the garage door and one wall exposed to the outside. It has living space above it and on the other side of the two remaining walls. If there's a week of -20 C, the garage will be near -10 and water will certainly freeze. Garage doors are generally poorly insulated, if at all (mine isn't) and have massive air leaks to the outside world (mine does). If your garage has more walls exposed to the outside, or the roof exposed to the outside, it will get closer to the outside temperature ...
Just drain your cooling system and put a piece of tape over the ignition switch with "NO COOLANT" written on it as a reminder. Then you don't have to worry about it. It is not worth the risk to do otherwise.