magit-run-git-gui-blame: fix when linenum is missing

We can't guarantee that we'll always know which line number
we want to center the `git gui blame' process around.  This
fixes an error caused by attempting to pass nil as one of
the arguments to `start-file-process'.

Message-by: Sylvain Chouleur <sylvain.chouleur@intel.com> :-)
Reported-by: Sylvain Chouleur <sylvain.chouleur@intel.com>