The Linux network monitor skipped every route update whose Dst was not
nil, treating a nil Dst as the default route. Since the bump to
vishvananda/netlink v1.3 (v0.43.0) the default route arrives as an
explicit 0.0.0.0/0 or ::/0, so every default route change was filtered
out and the monitor never fired on Linux, no matter how the underlay
moved.
Treat both a nil Dst and a zero-length prefix as the default route.
