[BUG] SofaMetaDataHandler uses raw .equals on possibly-null fields → NPE

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-sofa/src/main/java/org/apache/shenyu/plugin/sofa/handler/SofaMetaDataHandler.java:39,43`
- description: `exist.getServiceName().equals(metaData.getServiceName())` and `exist.getRpcExt().equals(metaData.getRpcExt())` without null-safety. `MetaData.rpcExt` defaults to null. Unlike `AbstractDubboMetaDataHandler` which uses `Objects.equals(...)`, the sofa handler NPEs if a registered metadata has null `rpcExt`/`serviceName`.
- impact: A metadata re-handle event for a sofa path whose `rpcExt`/`serviceName` is null throws NPE.
- suggested_fix: Use `Objects.equals(...)` (as the dubbo handler does).
- confidence: Medium
- related_existing: none

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Inspect shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-sofa/src/main/java/org/apache/shenyu/plugin/sofa/handler/SofaMetaDataHandler.java at lines 39 and 43, then compare the null-safe handling in AbstractDubboMetaDataHandler. Verify that a metadata re-handle event with null rpcExt or serviceName no longer throws an NPE.

Written by the indexing model from the issue text.