libstdc++: Fix visit<void>(v) for non-void visitors [PR106589]
The optimization for the common case of std::visit forgot to handle the
edge case of passing zero variants to a non-void visitor and converting
the result to void.
libstdc++-v3/ChangeLog:
PR libstdc++/106589
* include/std/variant (__do_visit): Handle is_void<R> for zero
argument case.
* testsuite/20_util/variant/visit_r.cc: Check std::visit<void>(v).