fix(chain-spec): ChainSpecBuilder with object as default genesis (#4345)

The current `json_patch::merge` function will replace the default
configuration with the builder action if the action is not a JSON
Object, which means, if not configured, the current code will try to use
`Value::Null` as the config, which (almost?) always fails, making it
mandatory to configure `with_genesis_config...`.

This change uses a JSON Object as the default values, which means the
code will continue using the default config if none is given.

Co-authored-by: Bastian Köcher <[email protected]>