Update JsValue::to_json to support undefined

[jamesthurley]

This Pull Request fixes/closes #3842.

Personally I've found JsValue::to_json to be fairly useless in its current implementation, as it will panic if the data structure being converted contains undefined, and I often don't have control over this.

The PR changes the following:

The JsValue::to_json method no longer panics if the structure contains undefined.

If undefined is encountered it will:

• Omit the property if it is part of an object.
• Replace with null if undefined is an element of an array. I don't love this, but couldn't think of a better solution.
• Return None if to_json is being called on JsValue::undefined().

Because of the last point there is a breaking change: JsValue::to_json now returns JsResult<Option<Value>> instead of JsResult<Value>.

To avoid a breaking change we could perhaps introduce a new method try_to_json instead, and have the original to_json call try_to_json and panic if None is returned.

On the other hand, the new behaviour forces people to think about handling undefined, which will probably prevent unexpected panics in the future.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 52.59%. Comparing base (6ddc2b4) to head (882fb4e).
Report is 396 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/value/conversions/serde_json.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4212      +/-   ##
==========================================
+ Coverage   47.24%   52.59%   +5.34%     
==========================================
  Files         476      487      +11     
  Lines       46892    51965    +5073     
==========================================
+ Hits        22154    27330    +5176     
+ Misses      24738    24635     -103     

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[jedel1043]

Replace with null if undefined is an element of an array. I don't love this, but couldn't think of a better solution.

I think this is fine. The specification already does this, so it is nice to have parity with it.

[jedel1043]

This should probably be removed.

[jamesthurley]

Done, good catch.

[jedel1043]

Looks good! Thank you for the contribution!