Compiler: fix sizeof Nil and other empty types

[asterite]

Fixes #7644

[asterite]

I forgot to explain this about more: Pointer(T).malloc is implemented as a primitive, and there we delegate the allocated size to LLVM using its sizeof function, which for Nil (and other "empty" types returns 0). So mallocing a Nil pointer always asks for 0 memory. But when doing Pointer#clear we use our sizeof, which calls LLVM's once and then would change it to 1 if it's zero. That caused a segfault trying to access something past that pointer. I guess I made it like that for the Pointer(Void) and sizeof(Void) case, but I generalized too much.

I'm still not sure about us being able to do Pointer(Void) because it's just so it looks like what you can do with C, but at least this fixes the issue without introducing any breaking change.