child_process: exit spawnSync with null on signal

This commit sets the spawnSync() exit code to null when the
child is killed via signal. This brings the behavior more in
sync with spawn().

Fixes: https://github.com/nodejs/node/issues/11284
PR-URL: https://github.com/nodejs/node/pull/11288


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>