Null safety is required for cluster pool creation

When creating a ClusterPool, null safety is required for the cluster and group names.
Follow [this](https://github.com/validatedpatterns/common/pull/472#discussion_r1572200684) review for further information. The target source file is [here](https://github.com/validatedpatterns/common/blob/main/acm/templates/provision/clusterpool.yaml).