Null objs

[abloomston]

This PR fixes the following scenario:

JSON schema:

{
    "type": "object",
    "properties": {
    "foo": {
        "oneOf": [
        {
            "type": "null"
        },
        {
            "type": "array",
            "items": {
            "type": "number"
            }
        }
        ]
    }
    }
}

JSON object:

{
    "foo": null
}

JSON path: $.foo.[0]

[abloomston]

@kennknowles note that travis ci build is failing for some reason unrelated to my change. I reproduced the travis ci failure at eb23b1e in my fork, cf: https://travis-ci.org/abloomston/python-jsonpath-rw/builds/83227690. If you restart this travis ci build it should fail: https://travis-ci.org/kennknowles/python-jsonpath-rw/builds/73743345.

[kennknowles]

Thanks!