libstdc++: Fix ranges::stable_sort handling of null buffer [PR123180]

libstdc++: Fix ranges::stable_sort handling of null buffer [PR123180]

The logic of the null pointer check got reversed when converting the
std::stable_sort code for ranges::stable_sort.

libstdc++-v3/ChangeLog:

	PR libstdc++/123180
	* include/bits/ranges_algo.h (__stable_sort_fn::operator()): Fix
	sense of null check. Replace typedef with alias-declaration.
	* testsuite/25_algorithms/stable_sort/123180.cc: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>