cb-cmds: empty value for next-server parameter is accepted which causes misconfiguration

It cases a failure during lease assignment:

2019-04-17 07:52:58.700 DEBUG [kea-dhcp4.bad-packets/6301] DHCP4_PACKET_DROP_0007 [hwtype=1 08:00:27:f9:dd:64], cid=[no info], tid=0xf52d94: failed to process packet: Failed to convert string to address '': Invalid argument

If value and its format can be validated upfront during accepting configuration in cb-cmds it should be done to avoid extra effort of debugging misbehaviour of Kea.

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