[BUG] AbstractNodeDataChangedListener.onSelectorChanged builds config-key prefix with raw getNamespaceId() (no SYS_DEFAULT fallback) — selectors invisible when namespaceId null

## Description
Each plugin's `doExecute` calls `CACHED_HANDLE.get().obtainHandle(key)` and immediately dereferences the result without a null check. `CommonHandleCache.obtainHandle(key)` returns `cached.get(key)` which is `null` on cache miss. The cache miss occurs when a rule is created with a null/empty `handle` field — every data handler guards the cache write with `Optional.ofNullable(ruleData.getHandle()).ifPresent(...)`, so a rule with no handle JSON is never cached but CAN still be matched by `AbstractShenyuPlugin.execute` (matching is condition-based, not handle-based).

## Location
- `shenyu-plugin/shenyu-plugin-fault-tolerance/shenyu-plugin-hystrix/src/main/java/org/apache/shenyu/plugin/hystrix/HystrixPlugin.java:59-60`
- `shenyu-plugin/shenyu-plugin-fault-tolerance/shenyu-plugin-resilience4j/src/main/java/org/apache/shenyu/plugin/resilience4j/Resilience4JPlugin.java:66-67`
- `shenyu-plugin/shenyu-plugin-fault-tolerance/shenyu-plugin-sentinel/src/main/java/org/apache/shenyu/plugin/sentinel/SentinelPlugin.java:59-60`
- `shenyu-plugin/shenyu-plugin-fault-tolerance/shenyu-plugin-ratelimiter/src/main/java/org/apache/shenyu/plugin/ratelimiter/RateLimiterPlugin.java:72-74`
- `shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/cache/CommonHandleCache.java:42-44`

## Impact
Every request matching a rule that was created without a handle (or whose handle was cleared) receives an unhandled `NullPointerException` / 500. A single misconfigured rule takes down all traffic matching it.

## Suggested fix
Add a null guard at the top of each `doExecute`, e.g. `if (Objects.isNull(handle)) { return chain.execute(exchange); }`, mirroring the guard added in `ContextPathPlugin` and `MockPlugin` (line 52-54).

## Related existing
Distinct from #6657 (GeneralContextPlugin NPE) — that issue was filed only for `GeneralContextPlugin.java:54`. These are four separate plugins with separate caches and separate `doExecute` implementations, all of which lack the guard that #6657 added for `GeneralContextPlugin` only.

Start with CommonHandleCache.java and inspect the doExecute methods in HystrixPlugin.java, Resilience4JPlugin.java, SentinelPlugin.java, and RateLimiterPlugin.java. Compare their handling with ContextPathPlugin and MockPlugin, then verify that rules without a handle no longer produce an unhandled exception and that matching requests continue through the chain.

Written by the indexing model from the issue text.