Broken protobuf generation with Optional Vector

When an IDL includes a hashMap type, the generated protobuf is invalid.
```json
{
  "name": "creators",
  "type": {
    "option": {
      "vec": {
        "defined": "Creator"
      }
    }
  }
},
```

```
	optional repeated Creator creators = 5;
```

`optional` cannot be used with `repeated`