fix: avoid NPE in IngressReconciler.getZookeeperUrl when zookeeper service has no endpoints (#6865)

[juicewcode]

Fixes #6865

Summary

• For a Dubbo/Sofa ingress that uses a zookeeper service name as the register address,
  IngressReconciler.getZookeeperUrl fetched V1Endpoints from the lister. When the service has no Endpoints yet,
  v1Endpoints is null and v1Endpoints.getSubsets() threw an NPE during reconcile, blocking route configuration. This PR
  adds a null check on v1Endpoints: when no Endpoints are found it logs a clear message and falls through to the
  existing ShenyuException fallback (the same path as empty subsets), so no NPE is thrown.

• Added IngressReconcilerZookeeperUrlTest with two cases — testReconcileWithoutZookeeperEndpoints verifies
  the graceful ShenyuException fallback instead of an NPE (regression); testReconcileWithZookeeperEndpoints verifies
  the service name is still resolved to the Pod IP (zookeeper://127.0.0.1:2181) and written into the dubbo plugin config
  (normal path).

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