cmdlib: Fix “unpack non-sequence” error

The callback is expected to return a two-valued tuple.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

lib/cmdlib.py

@@ -12190,6 +12190,8 @@ class LUInstanceSetParams(LogicalUnit):
     private.params = new_params
     private.filled = new_filled_params
+    return (None, None)
+
   def CheckPrereq(self):
     """Check prerequisites.