Empty response schema generates an exception "Unknown media type schema type"

Describe the bug

"responses": {
  "400": {
    "description": "There was a problem with the data sent with the request.",
    "content": {
      "application/json": {},
      "application/xml": {}
    }
  }
}

Not having a schema on the returned content will generate an "Unknown media type schema type" exception. This is valid schema and many code first schema generators create this type of output.

Desktop (please complete the following information):

• OS: macOS 10.13.7
• Python Version: 3.10.15
• openapi-python-client version 0.5.0

首先使用包含空 application/json 和 application/xml content 对象的 OpenAPI 响应示例重现该问题。跟踪会引发 "Unknown media type schema type" 的媒体类型 schema 处理；当不带 schema 的有效响应能够成功生成且不会出现该异常时，即表示完成。