Enabling testing for a pointer that references a nonexistent value

[zmarois]

Thanks for hosting this repo. This is the same as java-json-tools#41

We are trying to write a JSON Patch test operation that asserts the value at a path is not set.

Per the "Error Handling" section of the RFC on JSON Pointers:

Error conditions include, but are not limited to:

• Invalid pointer syntax
• A pointer that references a nonexistent value

This specification does not define how errors are handled. An application of JSON Pointer SHOULD specify the impact and handling of each type of error.

For example, some applications might stop pointer processing upon an error, while others may attempt to recover from missing values by inserting default ones.

A JSON Patch test operation can only test that evaluating a JSON pointer results in a specified value. I would like to consider that processing a JSON Pointer for the application of a JSON Patch test operation would evaluate a nonexistent value as null, at least when comparing to null.