allow null as well as '{}' for empty objects in json

[kryton]

certain json parsers (serde_json in rust) convert no-field empty structs as null, instead of '{}'. This patch allows for both ways to seen as valid. (for example) ["get_status_request_v0",null] as well as ["get_status_request_v0",{}]

[kryton]

NB:.. serde_json still won't fully work with variants. as it converts 'enums' as objects, not arrays.. eg {"get_status_request_v0":null} is what it would generate that's a non-compatible change .. so won't be submitting a PR for that bit