[BUG] - result may be null, causing NPE

[liljdd]

Steps to Reproduce

1. env:jdk21+swagger3
2. a java bean has "Exception" property,eg.@DaTa
   public class Blog {
   private Exception exception;
   }
3. the bean to swagger's Schema object
4. runtime errors：java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.media.Schema.get$ref()" because "result" is null

Expected Behavior

Rectify the NPE.

Servicecomb Version

3.2.0

Additional Context

No response

[liubao68]

Exception 应该无法使用 swagger 序列化表示吧？ 你期望他的swagger长什么样了？