hostcfg: Treat omitted field and null in the same way

E.g., not specifying "host_ip" and explicitly setting "host_ip":null are equivalent, see accepted proposal for details:

https://git.glasklar.is/system-transparency/project/documentation/-/blob/main/proposals/2024-01-31-hostcfg-optional-keys.md?ref_type=heads

Note: there's a link to a drafty implementation in the above proposal. See the early feedback in See early feedback in: !149 (closed) when MR:ing this separately.