persistence: avoid setting empty storageClassName

[lindhe]

Description

Without the additional logic introduced in this change, the storageClassName field would by default be set to empty string.

For systems with a default storage class configured, setting storageClassName to the empty string would be OK during installation since the storage class controller will set the storageClassName afterwards.

This, however, does not work for upgrades because then the storageClassName of the live object is already set to something and then we try to change that immutable field to the empty string.

This change ensures that the storageClassName is only set if we specify persistence.storage_class.

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