The heater should be drained at one time or another for cleanliness purpose. Nothing will happen to the heater as long as the heater is not connected to the power source. Just make sure that after draining, it gets enough air then get it ready for use.
Do not drain tank unless there is risk that it will freeze. You can turn thermostat down to 'vacation' setting and that will keep water heated to 50 degrees F. If tank is empty of water, then anode rod cannot protect steel tank from rusting. http://waterheatertimer.org/Replace-anode-rod.html