{Compute} az disk-encryption-set update: Set source vault to None when --source-vault is not input (#31678)

Author: yanzhudd

src/azure-cli/azure/cli/command_modules/vm/operations/disk_encryption_set.py

@@ -65,3 +65,8 @@ def pre_operations(self):
         if has_value(args.source_vault):
             vault = args.source_vault.to_serialized_data()
             args.source_vault = vault if is_valid_resource_id(vault) else KV_RID_TEMPLATE.format(self.ctx.subscription_id, args.resource_group, vault)
+
+    def pre_instance_update(self, instance):
+        args = self.ctx.args
+        if not has_value(args.source_vault):
+            instance.properties.active_key.source_vault = None