[BUG] SelectorDataRefresh.refresh() NPE on null data / no-op on empty data (HTTP sync)

- Severity: High
- Location:
`shenyu-sync-data-center/shenyu-sync-data-http/src/main/java/org/apache/shenyu/sync/data/http/refresh/SelectorDataRefresh.java:71-81`
-
Description:
`if (CollectionUtils.isEmpty(data)) { ...; data.forEach(pluginDataSubscriber::unSelectorSubscribe); pluginDataSubscriber.refreshSelectorDataAll(); }`. `CollectionUtils.isEmpty(null)` returns true, so the branch is entered with `data == null`, then `data.forEach(...)` throws `NullPointerException`. If `data` is an empty list, `forEach` is a no-op (unsubscribe nothing) and the clear intent is not achieved. The ordering is also backwards (`forEach` before `refreshSelectorDataAll`). Compare `PluginDataRefresh` which correctly calls `refreshPluginDataAll()` first and returns before iterating.
-
Impact:
HTTP sync refresh with an empty/null selector group crashes the long-polling task (NPE) or silently leaves stale selectors in cache.
-
Suggested fix:
Move `pluginDataSubscriber.refreshSelectorDataAll()` to before the empty check, and return early when empty (same pattern as `PluginDataRefresh`).
-
Confidence: High

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/00-consolidated-critical-high.md`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._

Start at shenyu-sync-data-center/shenyu-sync-data-http/src/main/java/org/apache/shenyu/sync/data/http/refresh/SelectorDataRefresh.java:71-81 and compare its flow with PluginDataRefresh, which is identified as the correct pattern. Verify that null and empty selector data clear existing subscriptions without throwing, and that refreshSelectorDataAll runs before the empty check.

Written by the indexing model from the issue text.