OpenAPI 3 empty object responses generate invalid Go clients

Empty object responses are causing gnostic to produce invalid go stanzas:

Given a schema response such as:
```
responses:
'200':
description: Any endpoint that yields "{}"
content:
application/json:
schema:
type: object
```

gnostic produces a client method whose result is:

`result := &interface{}{}`

I've also tried adding the implicit: `properties: {}`, but the generated client includes the same bug (`expected ';', found '{'`).

More details can be found here, https://gist.github.com/displague/31700ba03009d1a183297949c54e7471 including the schema being used and the full client output.