vxlan.vlans list is invalid on nodes that have no VLANs

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When a node has no VLANs, the VXLAN module_post_transform hook skips the node without cleaning up the vxlan.vlans list. That might cause some interesting problems in configuration templates that do not check whether the related VLAN exists on the device.

From the "device data model should be consistent" perspective, we should remove vxlan.vlans attribute from nodes that have no vlans attribute.

Discovered by @jbemmel in #466