[ISSUE #995] Harden cluster page against null component lists for real clusters

[123123213weqw]

What is the purpose of the change

When a real RocketMQ cluster (configured via STUDIO_ROCKETMQ_NAMESRV_ADDR) is described by RealClusterProvider and no proxy is provisioned, the produced ClusterVO leaves proxies, tpsHistory and config as null. The cluster page dereferences them directly (c.proxies.length, cluster.config, summary counts), so a null payload throws a TypeError and the page goes blank.

This PR makes the payload safe on both sides:

• Backend: RealClusterProvider now defaults a live cluster's absent runtime collections to empty — proxies/tpsHistory as List.of(), config as an empty ClusterConfigVO — so the API never serializes JSON null for these component lists.
• Frontend: every brokers/proxies/nameServers/config access in the cluster page is guarded with ?? [] / ?? {}, and the summary counts are extracted into a pure countClusterComponents helper with unit tests.

Brief changelog

• server/.../cluster/broker/RealClusterProvider.java: default config and tpsHistory to safe empty values (proxies already defaulted to List.of()).
• web/src/pages/cluster/index.tsx: null-safe access to brokers, proxies, nameServers, selectedProxy.proxies, cluster.config; use countClusterComponents.
• web/src/pages/cluster/clusterStats.ts (new): pure summary-count helper with empty-collection fallbacks.
• web/src/pages/cluster/clusterStats.test.ts (new): unit tests for empty, mixed and null component lists.
• server/.../cluster/broker/RealClusterProviderTest.java: new test asserting non-null proxies/tpsHistory/config for a live cluster without a proxy.

Verifying this change

• mvn test -Dtest=RealClusterProviderTest — 4 tests pass (3 existing + 1 new).
• npm test (vitest run) — 391 tests pass, including the new clusterStats.test.ts.
• eslint src/pages/cluster — no errors.

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Closes #995

[123123213weqw]

Hello maintainers! This is my first pull request to this repo, so the CI workflow needs approval to run. @Aias00 @zhaohai could you please approve the workflow run for this PR? All tests pass locally (backend mvn test -Dtest=RealClusterProviderTest 4/4, frontend vitest run 391/391, eslint clean). Thanks!

[RockteMQ-AI]

LGTM — defensive improvement with proper error handling and test coverage.

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[RockteMQ-AI]

⚠️ Merge conflict detected

This PR has conflicts with the base branch and cannot be merged. Please rebase or merge the base branch into your branch and resolve the conflicts:

git fetch origin
git checkout fix/cluster-null-safety-995
git rebase origin/rocketmq-studio
# resolve conflicts, then:
git push --force-with-lease

This is a one-time reminder. Feel free to @mention me for a re-review after conflicts are resolved.

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