repart: deal with empty partition label sensibly
    
    libfdisk appears to return NULL when encountering an empty partition
    label, let's handle this sanely, and treat NULL and "" for the current
    label as the same, but for the new label as distinct: there NULL means
    nothing is set, and "" means an actual empty label.
    
    (cherry picked from commit be9ce0188ebb414319f0c003f805ea02b5eb473e)