sim: Add a NullSimObject.descendants function.

Null simobjects don't have any descendants.

Change-Id: Ia43a99056709f422c9c817c017912d23d689fb1e
Reviewed-on: https://gem5-review.googlesource.com/4844
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/python/m5/params.py

@@ -1716,6 +1716,10 @@ class NullSimObject(object):
     def clear_parent(self, old_parent):
         pass
 
+    def descendants(self):
+        return
+        yield None
+
     def __str__(self):
         return 'Null'