Don't call NetlinkCallbacks with nullptr on NLMSG_DONE.

No real callback uses this, and even the test code doesn't seem
to use it for anything useful.

Bug: 34873832
Test: bullhead builds, boots, rules flushed on netd restart
Test: netd_{unit,integration}_test pass
Change-Id: I195dd388864e9e596af9f4d08aee7b8ade078fb5