tests/sys/netgraph: Further CI fixes

I was trying to debug why this test is working locally but failing in CI.
While doing so I made some small changes to allow running it with set -e.

It turns out the problem is that find_iface does not return anything in
Jenkins, so all following tests fail with obscure error messages.
To handle this case exit early if $eth is empty.

Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D29340

(cherry picked from commit 7dd1f932c1f51bfe10da7bc8875879cdcdd40821)