For the master, the purpose of -m/--masters is to specify addresses
of other master nodes, since its own address is already known via
-b/--bind. Therefore, an empty value for -m/--masters is valid.
The user remains free to repeat the -b value in -m.

More generally, a node may choose to only specify master addresses
via -D/--dynamic-master-list, so the check that at least one master
address is specified is moved where the NodeManager is expected to be
initialized.