[DeLICM] Use polly::singleton to allow empty result.
isl_map_from_union_map cannot determine the map's space if the union_map
is empty. polly::singleton was designed for this case. We pass the
expected map space to avoid crashing in isl_map_from_union_map.
This fixes an issue found by the aosp buildbot. Thanks to Eli Friedman
for the reproducer.