libstdc++: Rename __detail::__maybe_empty_t alias template

libstdc++: Rename __detail::__maybe_empty_t alias template

The key property of this alias is not that it may be an empty type, but
that the type argument may not be used. The fact it's replaced by an
empty type is just an implementation detail.  The name was also
backwards with respect to the bool argument.

This patch changes the name to better reflect its purpose.

	* include/std/ranges (__detail::__maybe_empty_t): Rename to
	__maybe_present_t.
	(__adaptor::_RangeAdaptor, join_view, split_view): Use new name.