Map enum tuple variants with no contents as unitary enum variants

Map enum tuple variants with no contents as unitary enum variants

cbindgen maps `enum A { V() }` with a `struct A_V_Body {}` which
has different size on C and C++ (0 and 1 bytes). While this is
fine if a different variant has non-0 size as the union will be
correctly sized. However, there isn't a lot of reason to rely on
this when we can just have a unitary variant as well.