[BUG] Extended location API always returns empty `authentication` block even it's not specified in the request payload

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/b9765efcc5ea795d69aeb8826f78101b3a35c615/specification/extendedlocation/resource-manager/Microsoft.ExtendedLocation/stable/2021-08-15/customlocations.json#L648

### API Spec version

2021-08-15

### Describe the bug

`authentication` is an optional attribute, the user can omit this block when creating an extended location resource. But in the API response, it will always show up. If it's not specified by the user, it will be an empty block then.

### Expected behavior

`authentication` should not be returned as part of the response if it's not specified by the user in the request payload.

### Actual behavior

`authentication` always show up in the response even if it's not specified by the user.

### Reproduction Steps

Create an extended location resource without configuring `authentication` block, then intercept the response from the API, you can see an empty `authentication` block in the response.

### Environment

_No response_