v2 to v3 converter: support for x-nullable extension in formdata parameters

[jmini]

As demonstrated in #810 the x-nullable: <true or false> extension in OASv2 should be converted to nullable: <value> in OASv3.

This does not work for formData parameters like this input:

      parameters:
        - name: p1
          in: formData
          description: first description
          required: false
          type: string
          x-nullable: true

OAS3 output:

      requestBody:
        content:
          multipart/form-data:
            schema:
              properties:
                p1:
                  type: string
                  description: first description

[jmini]

Pull-Request to fix this issue: #821

[gracekarina]

Closed By #821