[BUG] RuleDataRefresh.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/RuleDataRefresh.java:71-81`
-
Description:
Identical bug to SYNC-3 but for rules. `data.forEach(pluginDataSubscriber::unRuleSubscribe)` is called when `CollectionUtils.isEmpty(data)` is true. NPE if null, no-op if empty.
-
Impact:
HTTP sync refresh with empty/null rule group crashes or leaves stale rules.
-
Suggested fix:
Call `refreshRuleDataAll()` first, return early when empty.
-
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)._

Open shenyu-sync-data-center/shenyu-sync-data-http/src/main/java/org/apache/shenyu/sync/data/http/refresh/RuleDataRefresh.java and inspect lines 71-81 in refresh(). Compare the empty and null rule-group paths with the suggested refreshRuleDataAll() ordering. Done means HTTP sync clears stale rules for empty data and does not throw an NPE for null data.

Written by the indexing model from the issue text.