[2.1.14] LuaQualityPrototype fields missing optional

[radorei]

For the documentation of LuaQualityPrototype, the fields previous and next appear as always readable, when in fact they can be nil.

For instance, in vanilla Space Age, prototypes.quality["normal"].previous is nil, and prototypes.quality["legendary"].next is nil.

[Bilka]

Thanks for the note, fixed for the next doc release.