As per the other string types, `?[:0]const u8` needs its own case
as otherwise it will raise an error about using `{}` with slices.

There's no reasonable workaround for this, as you would have to
either discount the use of the empty string value or manually
rework the string to be sentinel-terminated at runtime. It's
useful for passing build options to code making use of C libraries
that make strong use of sentinel-terminated arrays for strings.