Fixes #6484: Guard missing Dubbo ingress endpoints

[BobSong-dev]

Fixes #6484

Background

DubboIngressParser dereferences the Endpoints informer result before checking it. During ingress reconciliation, the referenced Service may exist before its Endpoints resource is available, causing parsing of a default or path backend to throw a NullPointerException.

Changes

• Return an empty upstream list and log when Endpoints are unavailable for a Dubbo default backend;
• Return an empty upstream list and log when Endpoints are unavailable for a Dubbo path backend;
• Add regression coverage for both missing-Endpoints paths.

Verification

• ./mvnw test -pl shenyu-kubernetes-controller -Dtest=DubboIngressParserTest -Dmaven.javadoc.skip=true -Drat.skip=true -Djacoco.skip=true: BUILD SUCCESS; 9 tests passed;
• git diff --check: passed;