[VertexAI] `GenerateContentCandidate.parts` is undefined

### Operating System

n/a

### Environment (if applicable)

n/a

### Firebase SDK Version

11.7.1

### Firebase SDK Product(s)

VertexAI

### Project Tooling

JS

### Detailed Problem Description

When using multimodal content generation with Gemini and streaming responses, the last chunk returned will have no parts, which causes an internal error.

There is currently no workaround for this issue.

### Steps and code to reproduce issue

```js
const model = getGenerativeModel(ai, { model: 'gemini-2.0-flash-exp', generationConfig: { responseModalities: ['TEXT', 'IMAGE'] }});
model.generateContentStream('generate an image of a dog'); // @firebase/vertexai: FirebaseError: AI: Part should have at least one property, but there are none. This is likely caused by a malformed response from the backend. (AI/invalid-content)
```