Allow deletion of resources with `AzureName` empty

[theunrepentantgeek]

Describe the current behavior

The AzureName of every resource should be set automatically by our webhook, but if that fails we can end up with resources where AzureName is empty. This results in an invalid ARM resource URL, resulting in an error when someone tries to delete the resource.

Describe the improvement

When AzureName is empty (and therefore when any generated ARM resource ID will be invalid by definition), skip the attempt to delete the Azure resource and allow deletion of the cluster resource to proceed. (This would have enabled easier recovery for #4285.)