Make root_mount_rel(9) ignore NULL arguments, like it used to before r313351.
It would be better to fix API consumers to not pass NULL there - most of them,
such as gmirror, already contain the neccessary checks - but this is easier
and much less error-prone.

One known user-visible result is that it fixes panic on a failed "graid label".

PR: 221846
MFC after: 2 weeks
Sponsored by: DARPA, AFRL