BUG: Self nested schemas result in null values.

When using self nested schemas with `@marshall_with`, the result in swagger for that schema appears null.

For instance:

```
class UserSchema(Schema):
id: fields.Int()
name: fields.String()
friends: fields.Nested(lambda: UserSchema(many=True))
```

Results in swagger as
`
{
id: Integer,
name: string,
friends: [ null ]
}
`

Reproduce the issue with the shown UserSchema using @marshall_with and inspect the Swagger schema-generation path for self-nested fields. Compare the generated friends schema with the expected nested UserSchema output; done when it no longer contains null.

Written by the indexing model from the issue text.