Marshal nil collections (maps, sets, lists) as empty

[nmiyake]

What happened?

• Define a Conjure object with a map, list and set field
• Marshal the empty Conjure object
• The JSON representation contains the key for each of the fields and the values are null

What did you want to happen?

• The JSON representation should contain a key for each of the field, but the value should be an empty map/list rather than null

Although the spec notes that Conjure clients/servers should coerce null to an empty value, in practice older Java implementations did not do this, and there are implementations like Typescript that do not have a serde layer and cannot properly handle this. There have also been conversations about updating the spec to require this behavior.

Updating the Go implementation to produce empty values rather than null will improve compatibility with other implementations.