Need to be able to override global options on a more specific level with an empty value [ISC-support #14907]
In general a string option cannot be empty.
There are a few standard options which can be empty, none are string option, some are IP address array with possibly no address.
But in this instance, the requirement is a special case in which there's a global option defined but at a lower level, this needs to be overridden with an empty value. (The converse of not having it defined globally, but inserting it specifically at a lower level).
If you try to define the option with a null string value, Kea does not accept the configuration:
ERROR [kea-dhcp4.dhcp4/38937] DHCP4_INIT_FAIL failed to initialize Kea server: configuration error using file '/etc/kea/kea-dhcp4.conf': option data does not match option definition (space: dhcp4, code: 252): failed to parse an option '252' holding string value - empty value is not accepted (/etc/kea/kea-dhcp4.conf:161:19)
Other DHCP servers can support this functionality.