Fix crash when whitelist file could not be found
    
    The 'whitelst' var referred to in the format string does not
    exist. 'self.whitelist' is the correct string to include here.
    Also, there's no point trying to read a whitelist from an
    empty string or any other false-y thing, so change the condition
    for when we just skip trying to read it a bit.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>