Fix nmos-cpp-registry and nmos-cpp-node startup without config JSON

[lo-simon]

Thank you to Niitsu, Keita-san from Sony, for reporting this issue, as no config JSON is provided to start nmos-cpp-registry and nmos-cpp-node.

Issue:

1. registry_model.settings/node_model.settings are default-constructed as null.
2. When no command-line arguments are supplied, no settings are loaded.
3. validate_registry_settings/validate_node_implementation_settings are called with a null JSON value.
4. Schema validation fails because the root type is not an object.

Solution:

• Initialise settings as an empty JSON object in base_model (nmos/model.h).

[jonathan-r-thorpe]

LGTM