[lua] Add nil check to black dragon title obtainment

[WinterSolstice8]

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What does this pull request do?

player can be nil in some cases, so add a nil check to prevent errors

Steps to test these changes

!hp 0 the dragon, get no errors

[almuth150]

I think this is a problem with quite a few mobs, actually (ie. every mob that gives a title, and other stuff that does stuff to player entities on death). Is there more cases of causing this kind of error, where the player is nil? If it's just from !hp 0 on the mob, that's not something that's seen in normal gameplay, but if this behaviour can occur during normal gameplay, that would be a problem.