title: JSON PATH Field NULL Checking Expression
body: <p>I have a json array like bellow :</p>
  
  <pre><code>{
  "store": {
      "book": [
          {
              "category": "reference",
              "author": "Nigel Rees",
              "title": "Sayings of the Century",
              "price": 8.95
          },
          {
              "category": "fiction",
              "author": "Evelyn Waugh",
              "title": "Sword of Honour",
              "price": 12.99,
              "likes": 1
          },
          {
              "category": "fiction",
              "author": "Herman Melville",
              "title": "Moby Dick",
              "isbn": "0-553-21311-3",
              "price": 8.99
          },
          {
              "category": "fiction",
              "author": "J. R. R. Tolkien",
              "title": "The Lord of the Rings",
              "isbn": "0-395-19395-8",
              "price": 22.99
          }
      ],
      "bicycle": {
          "color": "red",
          "price": 19.95
      }
  },
  "expensive": 10}
  </code></pre>
  
  <p>I was trying to find all books price which have no <strong>likes</strong> field. Is it possible to find using <a href="https://github.com/json-path/JsonPath" rel="noreferrer">Jayway Json Path library</a>. I am trying something like this <code>$.store.book[? (@.likes == null)].price</code> but i don't know whether there is anything like that?</p>