python: Don't try to bind a stat group to the NULL simobject.

python: Don't try to bind a stat group to the NULL simobject.

That is a SimObject like object which is used when a SimObject
parameter is purposefully left empty, vs. being accidentally left
empty through a typo or accidental ommission.

It doesn't have a getCCObject method, and attempting to use it anyway
causes gem5 to crash.

Change-Id: Ie86321fbdbcc41cf88b7009184423acd7b64484b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21059
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>