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 <git@kchr.de>

substrate/client/chain-spec/src/chain_spec.rs

@@ -325,7 +325,7 @@ impl<E, EHF> ChainSpecBuilder<E, EHF> {
  name: "Development".to_string(),
  id: "dev".to_string(),
  chain_type: ChainType::Local,
- genesis_build_action: GenesisBuildAction::Patch(Default::default()),
+ genesis_build_action: GenesisBuildAction::Patch(json::json!({})),
  boot_nodes: None,
  telemetry_endpoints: None,
  protocol_id: None,