ActiveDocs shows Error when a response body is blank (content-length: 0). This isn't depends on OpenAPI definition.

I tried to reproduce the issue with swagger-ui v3.35.1 (OpenAPI v3) and latest, but they handle blank response as expected.

How to reproduce:
1. Create a REST API that returns empty body
2. Open 3scale Admin Portal.
3. In the default "API" Product, configure a Backend that points to 1 as the API backend.
4. Promote the configuration to Production
5. Open ActiveDocs
6. Request to the API Backend from ActiveDocs

Expected Behavior: Detail field is blank.
Actual Result: Error message in Detail field.