Unable to filter for null values with JSONpath

I am attempting to apply some assertions against a JSON response where some of the field values are null (6/10 results in my example). In the example I can successfully filter using, to return the colorCategoryId values of the results that have colorCategoryId=7:

$.result.[?(@.colorCategoryId=='7')].colorCategoryId

if I use the same filter to check for null I get no results returned

$.result.[?(@.colorCategoryId=='null')].colorCategoryId

also the filter $.result.[?(@.colorCategoryId!='null')].colorCategoryId returns [null, 5, null, 7, null, 7, null, 3, null, null]???

How can I use a JSONpath match assertion to filter for null values? I have tried a variety of different things (including assigning a null value to a property and using a property expansion in the filter) but cannot find any way to filter for these items will null values. Thank you for your help.

Example JSON below:

{
   "isSuccess": true,
   "message": "",
   "exception": "",
   "result":    [
            {
         "id": 100744,
         "isActive": true,       
         "colorCategoryId": null,
         "assignees": [],
         "mediaTypeId": 1,
         "mediaTypeName": "OP"
      },
            {
         "id": 100867,
         "isActive": true,
         "colorCategoryId": 5,
         "assignees": [],
         "mediaTypeId": 2,
         "mediaTypeName": "OPT"
      },
            {
         "id": 101392,
         "colorCategoryId": null,
         "assignees": [],
         "mediaTypeId": 2,
         "mediaTypeName": "OPT"
      },
            {
         "id": 101395,
         "colorCategoryId": 7,
         "assignees": [],
         "mediaTypeId": 2,
         "mediaTypeName": "OPT"
      },
            {
         "id": 101734,
         "colorCategoryId": null,
         "assignees": [],
         "mediaTypeId": 1,
         "mediaTypeName": "OP"
      },
            {
         "id": 101738,
         "colorCategoryId": 7,
         "assignees": [],
         "mediaTypeId": 1,
         "mediaTypeName": "OP"
      },
            {
         "id": 101753,
         "colorCategoryId": null,
         "assignees": [],
         "mediaTypeId": 1,
         "mediaTypeName": "OP"
      },
            {
         "id": 101175,
         "colorCategoryId": 3,
         "assignees": [],
         "mediaTypeId": 2,
         "mediaTypeName": "OPT"
      },
            {
         "id": 101445,
         "colorCategoryId": null,
         "assignees": [],
         "mediaTypeId": 1,
         "mediaTypeName": "OP"
      },
            {
         "id": 101450,
         "colorCategoryId": null,
         "assignees": [],
         "mediaTypeId": 1,
         "mediaTypeName": "OP"
      }
   ],
   "pager":    {
      "itemCount": 35,
      "pageIndex": 1,
      "pageSize": 10,
      "pageCount": 4
   }
}