get_block_device_harder() returns == 0 if the fs is valid, but it is not backed by a single devno. (As opposed to returning > 0 if the devno is valid). Let's catch this case and log a clear message, and don't bother open the device in that case. This is mostly cosmetical, as either way, systemd-gpt-auto-generator doesn't work in scenarios like that. Prompted-by: #22504