When unmarshaling ChatCompletionChunk, the sdk doesn't check if the input is valid

Using the snippet below:
```
	chunk = openai.ChatCompletionChunk{}
	err := json.Unmarshal([]byte(`[]`), &chunk)
	fmt.Printf("%+v\n", err)
```

The err is nil and the chunk is successfully unmarshalled, though it will have empty value except that the `raw` field is `[]`.