AST generator: detect partial executed relations that become empty

AST generator: detect partial executed relations that become empty

In particular, detect those cases where they become empty
when intersected with the context at the innermost level.
At outer levels a similar test is already performed after
the call to isl_ast_build_compute_gist from add_node.

It may be better to detect these situations at an earlier stage,
but for now we just fix the immediate problem.

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