Do not check 'None' disk IDs for duplicates
Do not check 'None' disk IDs for duplicates

In case of 'None' logical or physical IDs, we don't need to check them
for duplicates. This case can happen for DRBD devices in case of newly
added disks, for example.

Reviewed-by: imsnah

andir/ganeti,mbakke/ganeti,mbakke/ganeti,dimara/ganeti,leshchevds/ganeti,apyrgio/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti/ganeti,ganeti/ganeti,grnet/snf-ganeti,grnet/snf-ganeti,dimara/ganeti,leshchevds/ganeti,mbakke/ganeti,apyrgio/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,andir/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,yiannist/ganeti,bitemyapp/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,andir/ganeti