Fix unspawned power creeps returning a weird null value for their TTL

[tiennou]

See #144 and the attached Discord discussion for details.

But in short, before the pcreep is spawned, o.ageTime is undefined — it gets defined by the spawnPowerCreep intent.
Which leads to the ticksToLive property subtracting the current tick value from undefined, which is NaN (no idea why, per the Discord discussion, this NaN would show up as a weirdly behaving null).

Fix the issue by checking that o.ageTime exists, and set it to undefined when the pcreep dies.