If JSON Key Exists Else

Hope you are having a great day! I am trying to return {'green color','road','street','yellow color'} which I was able to do with this function in python 3 but I only want it to return it if the key "CLASSES" exists in the dictionary "json_data". When I run the function with the following JSON code it works but when I run other code that does not have a key "CLASSES" I just want it to return None. How do I do that? When I give it other json data right now it shows the image at the bottom.

Here is the code:

def parse_classes(json_data):
    lst = list()
    for classes in json_data['images'][0]['classifiers'][0]:
        for item in json_data['images'][0]['classifiers'][0]['classes']:
            class1 = item['class']
            lst.append(class1)

    else:
        print("")
    lst = set(lst)
    return(lst)

Here is the json:

{
  "custom_classes": 0,
  "images": [
    {
      "classifiers": [
        {
          "classes": [
            {
              "class": "street",
              "score": 0.846,
              "type_hierarchy": "/road/street"
            },
            {
              "class": "road",
              "score": 0.85
            },
            {
              "class": "yellow color",
              "score": 0.872
            },
            {
              "class": "green color",
              "score": 0.702
            }
          ],
          "classifier_id": "default",
          "name": "default"
        }
      ],
      "image": "images/parisstreets/paris-streets-1.jpg"
    }
  ],
  "images_processed": 1
}

Image:

Thank you!

UPDATE: This is a followup question to Retrieve Multiple JSON Items Python