Fix startup NPE when a weakly dependent registry is unavailable (check=false)

[AryamannSingh7]

What is the purpose of the change?

Fixes #16178.

When a service subscribes to multiple registries and one of them is weakly dependent (check="false") and unavailable at startup, the application fails to start with:

java.lang.NullPointerException: Cannot invoke "org.apache.dubbo.registry.Registry.getUrl()" because "this.registry" is null
    at org.apache.dubbo.registry.ListenerRegistryWrapper.getUrl(ListenerRegistryWrapper.java:44)
    at org.apache.dubbo.registry.integration.RegistryDirectory.subscribe(RegistryDirectory.java:...)

This is a regression: Dubbo 3.2.0–3.2.4 start successfully, 3.2.5+ fail (also reproduced on 3.3.x).

Root cause

Under check=false, when a registry cannot be created (the registry is down), AbstractRegistryFactory#getRegistry swallows the exception and returns null. RegistryFactoryWrapper still wraps this null into a ListenerRegistryWrapper, so a null delegate is an expected state — ListenerRegistryWrapper already guards register / unregister / subscribe with if (registry != null).

However:

• getUrl / isAvailable / destroy / unsubscribe / isServiceDiscovery / lookup were not guarded.
• RegistryDirectory#subscribe computes a metrics cluster name via registry.getUrl().getParameter(...) unconditionally. That line was introduced in Support multi registries metrics key #12582 (released in 3.2.5), which is exactly why the NPE appears from 3.2.5 onward.

What changed

1. ListenerRegistryWrapper — complete the existing null-safety: getUrl returns null, isAvailable / isServiceDiscovery return false, destroy / unsubscribe become no-ops, and lookup returns an empty list when the delegate registry is null.
2. RegistryDirectory#subscribe — guard the metrics cluster-name computation against a null registry URL (no cluster name is reported when the registry is unavailable). RegistryEvent#toSubscribeEvent already tolerates a null name.

Verification

Added ListenerRegistryWrapperTest#testNullRegistryIsTolerated, which reproduces the exact NPE without the fix and passes with it. :dubbo-registry-api tests and spotless:check pass locally (JDK 21).

Checklist

• Make sure there is a GitHub issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction.
• Make sure GitHub actions can pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.85%. Comparing base (2626e28) to head (15d657b).

Files with missing lines	Patch %	Lines
...apache/dubbo/registry/ListenerRegistryWrapper.java	62.50%	1 Missing and 2 partials ⚠️
.../dubbo/registry/integration/RegistryDirectory.java	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16356      +/-   ##
============================================
+ Coverage     60.81%   60.85%   +0.03%     
- Complexity       30    11762   +11732     
============================================
  Files          1953     1953              
  Lines         89213    89217       +4     
  Branches      13460    13460              
============================================
+ Hits          54254    54290      +36     
+ Misses        29374    29349      -25     
+ Partials       5585     5578       -7     

Flag	Coverage Δ
integration-tests-java21	32.15% <30.76%> (-0.01%)	⬇️
integration-tests-java8	32.23% <30.76%> (+0.05%)	⬆️
samples-tests-java21	32.16% <30.76%> (+<0.01%)	⬆️
samples-tests-java8	29.74% <30.76%> (-0.08%)	⬇️
unit-tests-java11	59.05% <61.53%> (-0.01%)	⬇️
unit-tests-java17	58.56% <61.53%> (+<0.01%)	⬆️
unit-tests-java21	58.57% <61.53%> (+0.02%)	⬆️
unit-tests-java25	58.48% <61.53%> (-0.02%)	⬇️
unit-tests-java8	59.11% <66.66%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.