v3rpc: treat nil txn request op as error

Fixes #7889

etcdserver/api/v3rpc/key.go

@@ -253,8 +253,8 @@ func checkRequestOp(u *pb.RequestOp) error {
 			return checkDeleteRequest(uv.RequestDeleteRange)
 		}
 	default:
-		// empty op
-		return nil
+		// empty op / nil entry
+		return rpctypes.ErrGRPCKeyNotFound
 	}
 	return nil
 }