I would simply run a "regular" descale cycle first, then put it away.
The boiler is made of brass so it wont corrode, shouldnt be a problem. Many new machines sit for longer periods after manufacture and before sale to the end user, and they all have water in.
Or you could ofcourse, lend it to someone *:)
More specifically to your question however...
It is important to note that even if someone were to advise how to drain the boiler, you would need to be well aware that if you subsequently forget, to refill the boiler BEFORE switching the machine back on electrically (ie when it eventually comes out of storage), it would result in failure of the electric element (from being energised when dry) and requirement for subsequent expensive repair
I therefore wouldnt do it.