8244594: [BACKOUT] 8244523: Shenandoah: Remove null-handling in LRB expansion

[Roman Kennke]

I accidentally pushed some experimental changes with JDK-8244523, I want
to back this out, and re-push the patch that was reviewed.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8244594
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8244594/webrev.00/

Good?

Roman