dtrace: allow NULL interface pointer for ifinfo_t translator

This is similar to other translators and will be used in static
probes where the interface is not known.

Reviewed by: markj
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D43728

(cherry picked from commit e00fd24262945fdc7f16198c8abd977f743e66e4)