python: remove SimObject children on NULL assignment

Before this commit, on assigning NULL to an 'attr' that is a SimObject,
the corresponding children SimObject was not set to NULL.

This makes some SimObject being initialized in the c++ world
even if they are not needed.

This commit set the children to NULL in that case.

Change-Id: I2030d9d1b80c020fa28e50c0eca4f87b756763d6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51647
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/SimObject.py

@@ -1443,6 +1443,12 @@ class SimObject(object, metaclass=MetaSimObject):
                 e.args = (msg, )
                 raise
             self._values[attr] = value
+
+            # If we assign NULL to an attr that is a SimObject,
+            # remove the corresponding children
+            if attr in self._children and isNullPointer(value):
+                self.clear_child(attr)
+
             # implicitly parent unparented objects assigned as params
             if isSimObjectOrVector(value) and not value.has_parent():
                 self.add_child(attr, value)