[BUG] GeneralContextPlugin NPE on null cached handle

- severity: High
- files: `shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-general-context/src/main/java/org/apache/shenyu/plugin/general/context/GeneralContextPlugin.java:54`
- description: `doExecute` calls `obtainHandle(CacheKeyUtils.INST.getKey(rule))` and immediately dereferences `.isEmpty()` without a null check. `CommonHandleCache.obtainHandle` returns `null` on cache miss. Every other plugin in this set (ContextPathPlugin:55, RewritePlugin:68, etc.) guards with `Objects.isNull(handle)` before use; GeneralContextPlugin is the sole exception.
- impact: NullPointerException at runtime — request fails with 500 instead of passing through the chain.
- suggested_fix: Add `if (Objects.isNull(generalContextHandleMap) || generalContextHandleMap.isEmpty()) { return chain.execute(exchange); }` before line 54.
- confidence: High
- related_existing: none

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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 with shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-general-context/src/main/java/org/apache/shenyu/plugin/general/context/GeneralContextPlugin.java at line 54, then compare the null handling in ContextPathPlugin:55 and RewritePlugin:68. Check CommonHandleCache.obtainHandle for its cache-miss behavior. Done means a null or empty handle no longer causes an NPE and the request passes through the chain.

Written by the indexing model from the issue text.