libstdc++: Fix visit<void>(v) for non-void visitors [PR106589]

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).