topo: skip creating empty meshnet Topology CR for nodes with no links

[msupinodn]

Summary

When a node has no Links in its meshnet TopologySpec, kne currently still creates a Topology CR with spec: {}. meshnetd's Get(podName) RPC then reads the CR, fails the unstructured.NestedSlice(... "spec", "links") lookup with !found, logs could not find 'Link' array in pod's spec and returns an empty Pod. The CNI plugin sees NodeIp == "" and aborts the sandbox with the (misleading) error:

plugin type="meshnet" failed (add):
  meshnetd provided no HOST_IP address:  or HOST_INTF:

Every pod scheduled on a node with no links is then stuck in FailedCreatePodSandBox loops until kubelet's 4-minute deadline expires, and the only fix is to manually kubectl delete topology … for the offending CRs.

Fix

Skip CR creation when len(spec.Spec.Links) == 0.

The meshnet CNI plugin already handles "no Topology CR for this pod" gracefully (plugin/meshnet.go cmdAdd treats Get error as not a topology pod returning and chains through to the next plugin), so the resulting behaviour is identical to any other non-topology workload on the cluster.

This is a no-op for topologies whose nodes all have links, and substantially improves robustness for sparse / scale-test topologies where some nodes are intentionally isolated.

Test plan

• go vet ./topo/... clean.
• Reproduced end-to-end with a 200-node Alpine topology where ~30% of nodes had no random links — before this change those pods got stuck; after, they come up cleanly without the kubectl delete topology workaround.

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