grubby-bls: don't print rpm-sort error messages
    
    If there are no BLS snippets, an empty array of strings is passed to the
    rpm-sort tool and it will print an error that just confuses grubby users.
    
    Resolves: rhbz#1731924
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>