Handle Jackson Databind's 'MissingNode' null object better

[adityachatterjee42]

Jackson Databind uses com.fasterxml.jackson.databind.node.MissingNode as a 'null object'

Supplying the MissingNode singleton to JsonSchema.validate(...) leads to the following validation error message:
$: unknown found, {} expected

I suggest we explicitly handle MissingNode and return a more descriptive error message, or fail in a different way.

[justin-tay]

Can you provide the example and input data used? Are you sure it is MissingNode and not NullNode? I don't really see any issue with the message.

[stevehu]

Closing the issue as there is no action.