helm: fix allInOne.data.size and storageClass to use null defaults

Change size and storageClass from empty strings to null so the template
defaults (10Gi for size, cluster default for storageClass) will apply
correctly. Empty strings prevent the Helm | default function from working.

k8s/charts/seaweedfs/values.yaml

@@ -1183,8 +1183,8 @@ allInOne:
     type: "emptyDir"  # Options: "hostPath", "persistentVolumeClaim", "emptyDir", "existingClaim"
     hostPathPrefix: /mnt/data  # Path prefix for hostPath volumes
     claimName: seaweedfs-data-pvc  # Name of the PVC to use (for existingClaim type)
-    size: ""  # Size of the PVC (for persistentVolumeClaim type)
-    storageClass: ""  # Storage class for the PVC
+    size: null  # Size of the PVC (null defaults to 10Gi for persistentVolumeClaim type)
+    storageClass: null  # Storage class for the PVC (null uses cluster default)
     # accessModes for the PVC. Default is ["ReadWriteOnce"].
     # For multi-replica deployments, use ["ReadWriteMany"] with a compatible storage class.
     accessModes: []