db-schema/server-settings: fix fallback for custom LLM configs

[haraldschilly]

Description

This is rather trivial. If the datatype is expected to be a json of a dict, make the default an empty dict. Then, there are no special cases necessary if it isn't defined.

deployment: not necessary to do anything

Checklist:

• [ ] Testing instructions are provided, if not obvious
• [ ] Release instructions are provided, if not obvious