Don't try to backup partition table when the disc doesn't have one

I'm running backupninja of the stretch backport on a server with 4 discs in raid1. 2 discs have partition tables and the other 2 don't have partitions tables (/dev/sdc & /dev/sdd) these 2 last disc are in a array like this md4 : active raid1 sdd[1] sdc[0]

I get this error message when the job /etc/backup.d/30_main.sys runs.

Warning: The partition table for /dev/sdc could not be saved.
Warning: The partition table for /dev/sdd could not be saved.

Before trying to save the partition table, I think it should check if it exist and only if so, try to save it.

Since, I'm no coder, I can't offer much on the code.

I suspect it's around here. https://0xacab.org/riseuplabs/backupninja/blob/master/handlers/sys.in#L612

I see that the code around luksheaders is a bit more clever. https://0xacab.org/riseuplabs/backupninja/blob/master/handlers/sys.in#L635

Cheers,

Sébas