Allows to define empty response body for any status code

[joaopedrocampos]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Related issue: #434

OpenAPI generated schema

openapi: 3.0.3
info:
  version: 4.8.2
  title: fastify-swagger
components:
  schemas: {}
paths:
  /:
    get:
      responses:
        '204':
          description: No Content
        '503':
          description: Service Unavailable

Swagger generated schema

swagger: '2.0'
info:
  version: 4.8.2
  title: fastify-swagger
definitions: {}
paths:
  /:
    get:
      responses:
        '204':
          description: No Content
        '503':
          description: Service Unavailable

[coveralls]

Pull Request Test Coverage Report for Build 1017356547

• 0 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 1007475177:	0.0%
Covered Lines:	539
Relevant Lines:	539

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1017356547

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 1007475177:	0.0%
Covered Lines:	539
Relevant Lines:	539

---

💛 - Coveralls

[climba03003]

LGTM.

[mcollina]

lgtm