Attempt to fix Linux build

[wravery]

I haven't been able to reproduce the same build break locally, but this ought to fix the error that was reported in #301.

[wravery]

I was finally able to reproduce the original build break, and it happened with or without the first commit. I also needed to avoid constructing a temporary TypeData value on the stack just to assign it to _data. Instead, it should assign the std::variant option type directly to _data, which has already been constructed with a NullData in the constructor's initializer list.