Substitute MT_NULL with MT_RAY in Lua

As of !1973 (merged), P_SpawnMobj no longer replaces MT_NULL with MT_RAY, and instead aborts spawning MT_NULL mobj types. However, some Lua scripts, including [Silver the Hedgehog], expect MT_NULL to be spawnable.
This merge request replaces MT_NULL with MT_RAY in Lua's mobj-spawning functions, with a // TODO: 2.3: comment for making it show a Lua error instead for v2.3 (because usually, trying to spawn MT_NULL means that something has gone wrong, that you don't want to spawn anything and so don't need to call a spawning function, or that you should be using MT_RAY instead).
(SOC actions trying to spawn MT_NULL will still not spawn anything, though.)

TL;DR: Reverted Lua mobj-spawning to v2.2.13's behaviour. Not worth mentioning in v2.2.14's changelog.