Try to make the utils/listop functions return something sensible with empty input.

- Legacy-Id: 233

ietf/utils/listop.py

@@ -3,11 +3,17 @@ import operator
 def orl(list):
     """ Return the "or" of every element in a list.
     Used to generate "or" queries with a list of Q objects. """
+    if list:
         return reduce(operator.__or__, list)
+    else:
+        return None
         
 def flattenl(list):
     """ Flatten a list one level, e.g., turn
 	[ ['a'], ['b'], ['c', 'd'] ] into
 	[ 'a', 'b', 'c', 'd' ]
     """
+    if list:
         return reduce(operator.__concat__, list)
+    else:
+        return []