cdk8s-plus stateful_set.ts _filterPodSpecVolumes() deletes all volumes if there are no persistent templates

[war1025]

The last line of _filterPodSpecVolumes() should return the full list of volumes as the default case.

https://github.com/cdk8s-team/cdk8s-plus/blob/c14b130aa43958568a7de67f0ef31b053b6da602/src/stateful-set.ts#L253

Currently if you have no volumeClaimTemplates configured it will just delete all your volumes.