to_json() on a nested data structure containing undefined should not fail

[gschier]

Describe the bug

When calling to_json on an object like {foo: undefined}, it should result in {} and not panic. Here are some various behaviors (demo's using Node) which it would be good to follow:

> JSON.stringify({foo: undefined, bar: 1})
'{"bar":1}'
> JSON.stringify(undefined)
undefined
> JSON.stringify([1,2,undefined,4])
'[1,2,null,4]'

For context, I'm building a 3rd-party plugin system for Yaak using Boa and users may return a nested data structure that contain undefined values.

To Reproduce

Return an object from the JS context and call to_json on it.

export function callMe() {
  return { foo: undefined };
}

Expected behavior

The value should serialize to {} and not throw not yet implemented: undefined to JSON.

Build environment (please complete the following information):

• OS: macOS
• Version: 14
• Target triple: aarch64-apple-darwin
• Rustc version: rustc 1.77.2 (25ef9e3d8 2024-04-09)

Additional context

The current workaround is to return JSON.parse(JSON.stringify(objectContainingUndefineds)). However, as I'm using Boa to build a plugin system, I wouldn't expect users to have to do this themselves.

Another potential workaround is to recursively crawl the JSValue and remove undefined values from all the objects and arrays.

Thanks for taking the time to look into this!