Mod API Reference:Nullable
Just a signal that the variable can be nil in Lua. Nullable is not actually a proxy object in the Mod API, but still shows up on in the documentation as one.
Of course, everything in Lua can be nil, so this attribute may seem redundant. However, as a general rule, you can assume anything that's a primitive type like an integer or number will never be nil unless it's documented with this Nullable signal.