qf: Fix shell parameter expansion for the author name and email

A ':' was missing (should have been ':+'), but I think we want ':-', not
':+' ie we want to substitute if the parameter is undefined or empty:

  http://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion



Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>