Don't pass null arguments to hooks

If the change URL isn't available (e.g. because we don't have
gerrit.canonicalWebUrl configured) we cannot pass down the
--change-url flag to certain hooks.  Safeguard the code from
that failure case.

Bug: issue 641
Change-Id: Ib26ec8759e0090aadfc815c17fe7db0f299c640e
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed