cass-operator may crash if storageClassName is not set

[c3-clement]

What happened?

If Spec.StorageConfig.CassandraDataVolumeClaimSpec.StorageClassName is not set on the datacenter CR, StorageClassName will be a null pointer.
StorageClassName is getting dereferenced here https://github.com/k8ssandra/cass-operator/blame/master/pkg/reconciliation/reconcile_datacenter.go#L191 without a null check before.

If storageClassName is null, the operator will segfault.

What did you expect to happen?

I expect cass-operator to handle the case when storageClassName is not set by the user.
The operator should try to fetch the default storage class instead and call storageClass() with the name of the default storage class.

How can we reproduce it (as minimally and precisely as possible)?

I observed the issue when testing restoration.
I believe the issue will happen when storage expansion is triggered and when storage class name is not set.

I'm not sure why storage expansion got triggered during restoration.

cass-operator version

1.22

Kubernetes version

1.29

Method of installation

helm

Anything else we need to know?

No response

[burmanm]

Ah, PersistentVolume has type string and for PersistentVolumeClaimSpec they made it *string.

[burmanm]

Related: #123