My FIL had one that was decades old, it DID have a drain, however he never drained it. It corroded inside and out and one day he fired it up,pulled the hose out to fill up a tire just outside the door and the thing blew up. It had a pressure relief valve but the tank was so corroded it failed catastrophically at normal pressure.

Luckily he was not near it, as it was it caused him some hearing damage.

If in doubt, get rid of it and buy a newer larger one that has a drain and drain it every time it's used.

LPT: If the drain as a cheap drain cock type drain - like a cars radiator drain - buy a correct size 90° brass fitting and a ball valve with a nice handle on it. It makes the 'chore' of draining it sooo much easier. Just pop it open and let the air push out moisture for a few seconds when using it.