Be able to use nil as a compile-time known constant

[godalming123]

In my case, I'm working on a comiler, with the following code:

Type :: OrderedHashSetSlotRef

string_type :: Type{max(u32)}
int_type :: Type{max(u32) - 1}
uint_type :: Type{max(u32) - 2}
float_type :: Type{max(u32) - 3}
bool_type :: Type{max(u32) - 4}
invalid_type :: Type{max(u32) - 5}
unknown_type :: Type{max(u32) - 6}
type_type :: Type{max(u32) - 7}
any_type :: Type{max(u32) - 8}
max_index :: max(u32) - 9

CompileTimeValue :: union {
    StringLiteralValue,
    NumberValue,
    BoolValue,
    Type,
    CheckedFuncRef,
}

CheckedGlobalValue :: struct {
    type:  Type,
    value: CompileTimeValue,
}

uninitialised_global_value :: CheckedGlobalValue{unknown_type, nil} // This line of code causes an error