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