handle case where arg is None

Author: anon

qubesadmin/app.py

@@ -879,15 +879,15 @@ def qubesd_call(
                 raise qubesadmin.exc.QubesDaemonCommunicationError(
                     "{} not found".format(method_path)
                 )
-            assert arg is not None
             assert dest is not None
             command = [
                 "env",
                 "QREXEC_REMOTE_DOMAIN=dom0",
                 "QREXEC_REQUESTED_TARGET=" + dest,
                 method_path,
-                arg,
             ]
+            if arg is not None:
+                command.append(arg)
             if os.getuid() != 0:
                 command.insert(0, "sudo")
             (_, stdout, _) = self._call_with_stream(