It shouldn’t cause issues, since the water will remain in the heater without any pressure to push it out, there’s no difference between leaving it sitting with the valve open or closed.

Now, draining the tank could actually cause problems… But there’s not any reason for you to do that for what you’re trying to accomplish.

You should be able to turn off that cold water inlet valve instead. The pressure from the hot water tank comes from the cold water being pushed into it, so just turn off the valve and open a hot water faucet somewhere nearby to release any pressure that has already built up or may get through that crappy valve.