Fix invalid responses during promote

daemon-common/vm.py

@@ -4587,11 +4587,14 @@ def vm_worker_promote_mirror(
     vm_detail = get_list(zkhandler, limit=dom_uuid, is_fuzzy=False)[1][0]
 
     # Determine if there's a valid shared snapshot to send an incremental diff from
+    if destination_vm_detail:
         local_snapshots = {s["name"] for s in vm_detail["snapshots"]}
         remote_snapshots = {s["name"] for s in destination_vm_detail["snapshots"]}
         incremental_parent = next(
             (s for s in local_snapshots if s in remote_snapshots), None
         )
+    else:
+        incremental_parent = None
 
     current_stage += 1
     update(