Improve error when `null` occurs in entity json data

[john-h-kastner-aws]

Category

Other

Describe the feature you'd like to request

After discussion on cedar-policy/rfcs#37 (rejected), we have decided that we do not want to support null JSON values in entity JSON data. They are already an error, so no change is required there, but we should improve the error message.

The current error is data did not match any variant of untagged enum CedarValueJson

Describe alternatives you've considered

.

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change