BUG: Cannot update aks `ingressProfile.webAppRouting.dnsZoneResourceId` to empty nor null

According to the [document](https://github.com/MicrosoftDocs/azure-docs/blob/main/articles/aks/web-app-routing.md#configure-the-add-on-to-use-azure-dns-to-manage-creating-dns-zones), we can configure the add-on to use Azure DNS to manage creating DNS zones for aks cluster by the following az cli command:

```shell
az aks addon update -g -n --addon web_application_routing --dns-zone-resource-id=$ZONEID
```

Once time I set `$ZONEID` to a new created dns zone resource id and the update was successful, but when I set `$ZONEID` to empty string, after update I still can read original dns zone id in the response:

```json
"ingressProfile": {
"webAppRouting": {
"dnsZoneResourceId": "/subscriptions/xxxxxxxxx/resourceGroups/zjhe-aks-webrouting/providers/Microsoft.Network/dnszones/acctestzone8f7898a0.com",
"enabled": true
}
},
```

Start by reproducing the issue with `az aks addon update` using a valid DNS zone resource ID, then an empty or null value, and inspect the returned `ingressProfile.webAppRouting.dnsZoneResourceId`. Trace the Azure REST API specification for the AKS `webAppRouting` update operation and verify that clearing the value is represented and persisted correctly.

Written by the indexing model from the issue text.