title: Optional itemin a swagger yaml definition
body: <p>I have the following deinition in swagger.yaml</p>
  
  <pre><code>Content:
      type: object
      properties:
          text:
            type: string
          image:
              ref: "#/definitions/Image"
              allowEmptyValue: true
  </code></pre>
  
  <p>I get Additional properties not allowed: allowEmptyValue as an error</p>
  
  <p>How do I make image optional? ie might only be text no images</p>