libstdc++: Fix views::zip_transform constraints for empty range pack [PR111138]

libstdc++: Fix views::zip_transform constraints for empty range pack [PR111138]

Add missing move_constructible && regular_invocable constrains on functor type,
and is_object on functor result type for invocations of views::zip_transform
without range arguments.

	PR libstdc++/111138

libstdc++-v3/ChangeLog:

	* include/std/ranges (_ZipTransform::operator()):
	Create separate overload for calls with empty range pack,
	and add move_constructible, regular_invocable and
	is_object_v<invoke_result_t<...>>> constraints.
	* testsuite/std/ranges/zip_transform/1.cc: New tests

Reviewed-by: Patrick Palka <ppalka@redhat.com>
	Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>