Fix: null values throwing exception when traversing over while getting

[reckter]

Previously getting any value, where an object in the path was null, would throw an exception, while it being undefined would not.

Example:

const obj = {
   foo: null,
   baz: undefined
}

jsonpointer.get(obj, "/foo/baz") /// <--- This would throw
jsonpointer.get(obj, "/baz/foo") /// <---- This would not

The fix is simply also checking equality with null and then returning null (comparable to the undefined case, where it already returned null)

[marcbachmann]

Thanks for the fixes. I've introduced some better test assertions in #59 and rebased against it to get it in.
I also did a small fix to return undefined for .get('/nullValue/something')
IMO we can merge this once the other PR is merged.

[reckter]

Feel free to merge at will, I don't actually work with this library anymore, because the project is over where I used it, so you have full authority to merge and change at will :)