[BUG] `None ` value of `preferredAppGroupType` property is not valid for creating a Desktop Virtualization Host Pool

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/65da315d93751e6e06ffd42f44a77f5b1dac27bc/specification/desktopvirtualization/resource-manager/Microsoft.DesktopVirtualization/stable/2024-04-03/desktopvirtualization.json#L4960C11-L4960C13

### API Spec version

2024-04-03

### Describe the bug

Possible values for the property [preferredAppGroupType](https://github.com/Azure/azure-rest-api-specs/blob/65da315d93751e6e06ffd42f44a77f5b1dac27bc/specification/desktopvirtualization/resource-manager/Microsoft.DesktopVirtualization/stable/2024-04-03/desktopvirtualization.json#L4785C10-L4785C31) include `None`, but `None` is not allowed when creating a Desktop Virtualization Host Pool. See below for details.

### Expected behavior

Following the API spec `None` should be allowed.

### Actual behavior

The following error occurs:
```
"error": {
"code": "400",
"message": "ActivityId: 5abae8b1-e723-46d4-b505-038af70859a8 Error: The PreferredAppGroupType value is incorrect. Please provide a valid type: Desktop or RailApplications."
}
```

### Reproduction Steps

Create a Desktop Virtualization Host Pool and set the `preferredAppGroupType` to "None".

### Environment

_No response_

Start with the linked desktopvirtualization.json API specification at the preferredAppGroupType definition and review the 2024-04-03 version. Reproduce host pool creation with preferredAppGroupType set to "None", then determine whether the specification or service behavior needs correction; done means the documented allowed values match the observed API behavior.

Written by the indexing model from the issue text.