Stateful ProcessGroups can no longer be run without PVCs (Operator 1.35+)

Hi all!
The latest Operator release 1.35 will no longer support stateful processGroups to be created without PVCs.
This change was motivated when investigating some logic which allowed stateful processes to not have PVCs when a default spec was specified that did not have a volume or storage resource requests. We felt that it was more likely for someone to do this by accident than on purpose, and that stateful processes with no PVCs would not work in either case.
The code changes can be viewed in this PR here.
Please let us know here and/or file issues if this change is problematic for your use cases.