dot/emacs (bbdb-canonicalize-net-hook): Handle `nil' address.

dot/emacs (bbdb-canonicalize-net-hook): Handle `nil' address.

It turns out that, despite the documentation not mentioning this at all,
that the hook is allowed to return `nil' to suppress insertion of the
address into the database at all.  The only snag is that `nil' is
/still/ sent back to the hook to check for convergence.  So notice this
and return `nil' back, since it's not like we're going to be able to
turn it into anything more useful.