Print empty output of `dep ls`

Lack of printing empty result was especially confusing in column UI
(console wasn't opened and user could think that command wasn't
executed).

topydo/commands/DepCommand.py

@@ -119,9 +119,8 @@ class DepCommand(Command):
                 todo = self.todolist.todo(number)
                 todos = self.todolist.parents(todo)
             else:
-                self.error(self.usage())
+                raise InvalidCommandArgument
 
-            if todos:
             sorter = Sorter(config().sort_string())
             instance_filter = Filter.InstanceFilter(todos)
             view = View(sorter, [instance_filter], self.todolist)