addVoiceFocus recreates reference on empty device string causing infinite rerender when calling inside passive effect

### What happened and what did you expect to happen?

If the device passed to `addVoiceFocus` is an empty string, the function still calls:

```js
const vf = await transformer?.createTransformDevice(device);
if (vf) {
logger.info('Created a new Amazon Voice Focus transform device.');
setVoiceFocusDevice(vf);
return vf;
}
```

This causes `vf` to be created and subsequently `setVoiceFocusDevice(vf)` is invoked, which leads to a new reference for `addVoiceFocus` on every render.

If addVoiceFocus is called inside an effect and included as a dependency, this results in an infinite rerender.

https://github.com/user-attachments/assets/a73a5307-2f27-4656-b182-3be1fe326bbf

**Recommendation:**
- Add an early exit: don’t call transform if device is empty.
- Memoize addVoiceFocus so its reference remains stable when device doesn’t change.

### Have you reviewed our existing documentation?

- [x] Amazon Chime SDK for JavaScript GitHub [issues](https://github.com/aws/amazon-chime-sdk-js/issues)
- [x] Amazon Chime SDK React Components Library GitHub [issues](https://github.com/aws/amazon-chime-sdk-component-library-react/issues)
- [x] Storybook [documentation](https://aws.github.io/amazon-chime-sdk-component-library-react/?path=/story/introduction--page)
- [x] README [page](https://github.com/aws/amazon-chime-sdk-component-library-react#readme)
- [x] React SDK meeting [demo](https://github.com/aws-samples/amazon-chime-sdk/tree/main/apps/meeting#readme---react-meeting-demo)

### Reproduction steps

Steps to Reproduce:

- Call `addVoiceFocus` with an empty string as the device.
- Observe that `createTransformDevice` is invoked.
- Include `addVoiceFocus` in a React effect dependency array.
- Notice infinite rerendering.

### Amazon Chime SDK React Components Library version

^3.11.0

### What browsers are you seeing the problem on?

Chrome dev, comet

### Browser version

Version 143.0.7475.7

### Device Information

MacBook Pro; 14.1 (23B74)

### Meeting and Attendee ID Information.

_No response_

### Browser console logs

ConsoleLogger.js:79 2025-10-20T10:23:57.861Z [INFO] ChimeSDKReactComponent - Created a new Amazon Voice Focus transform device.

ConsoleLogger.js:79 2025-10-20T10:23:58.083Z [INFO] ChimeSDKReactComponent - Created a new Amazon Voice Focus transform device.

ConsoleLogger.js:79 2025-10-20T10:23:58.339Z [INFO] ChimeSDKReactComponent - Created a new Amazon Voice Focus transform device.

ConsoleLogger.js:79 2025-10-20T10:23:58.879Z [INFO] ChimeSDKReactComponent - Created a new Amazon Voice Focus transform device.

ConsoleLogger.js:79 2025-10-20T10:23:59.029Z [INFO] ChimeSDKReactComponent - Created a new Amazon Voice Focus transform device.

### Add any other context about the problem here.

_No response_

Start at the addVoiceFocus implementation and reproduce the empty-string case described in the issue, including calling it from an effect dependency array. The fix is done when an empty device does not create a transform device and addVoiceFocus does not change reference solely because the device remains unchanged, eliminating the infinite rerender.

Written by the indexing model from the issue text.