isl_basic_set_recession_cone: handle empty input explicitly
isl_basic_set_recession_cone: handle empty input explicitly

The internal implementation of isl_basic_set_recession_cone
destroys the canonical representation of empty basic sets and
turns it into the representation of a universe basic set.
While the result would still be marked empty, any function
that only looks at the constraints of the result would
end up using the wrong constraints.
Handle empty basic sets separately in order to preserve
the (canonical) representation.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl