libstdc++: Add missing initializers for __maybe_present_t members [PR119962]

libstdc++: Add missing initializers for __maybe_present_t members [PR119962]

Data members of type __maybe_present_t where the conditionally present
type might be an aggregate or fundamental type need to be explicitly
value-initialized (rather than implicitly default-initialized), so that
default-initialization of the containing class always results in an
completely initialized object.

	PR libstdc++/119962

libstdc++-v3/ChangeLog:

	* include/std/ranges (join_view::_Iterator::_M_outer): Initialize.
	(lazy_split_view::_OuterIter::_M_current): Initialize.
	(join_with_view::_Iterator::_M_outer_it): Initialize.
	* testsuite/std/ranges/adaptors/join.cc (test15): New test.
	* testsuite/std/ranges/adaptors/join_with/1.cc (test05): New test.
	* testsuite/std/ranges/adaptors/lazy_split.cc (test13): New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>