isl_scheduler.c: extract_edge: do not add edges that turn out to be empty

isl_scheduler.c: extract_edge: do not add edges that turn out to be empty

Even though extract_edge should only be called on non-empty
dependence relations, the relation may become empty after
intersection with the constraints used during compression.

Adding an empty edge is not usually a problem because
graph_has_edge will ignore such edges.
However, for edges of types that can appear multiple
times between the same pair of nodes, an empty edge
may shadow a non-empty edge, in which case the non-empty
edge(s) would be ignored.

Refrain from adding empty edges to avoid this potential problem.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>