metric is nil, instead of EnumType_INVALID.
This happened because ThemeMetric was setting the value to its default, eg.
EnumType(), when LuaHelpers::FromStack returned false. This behavior doesn't
make sense in the general case, since EnumType() results in an arbitrary
value (the first one) rather than Invalid. Every FromStack function sets
a reasonable default on invalid data, anyway, and only RageColor and enums can
return false in any case.
This fixes CustomDifficulty when CourseType = nil.
Glenn Maynard
143b96d5fa