[BUG] - result may be null, causing NPE

### 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_

Reproduce the failure with JDK 21, Swagger 3, and a Java bean containing an Exception property, then capture the full stack trace to identify the schema-conversion entry point. Trace the conversion that produces a null Schema and verify that the same bean converts without a NullPointerException; the issue does not name a source file or test to run.

Written by the indexing model from the issue text.