[BUG] springcloud-gateway sentinel 1.8.6版本报空指针异常

## Issue Description

Type: *bug report*

### Describe what happened

### Describe what you expected to happen

### How to reproduce it (as minimally and precisely as possible)

1. springcloud-gateway 3.1.4版本使用sentinel 1.8.6版本运行时会报空指针异常
2. sentinel相关依赖如下所示：


com.alibaba.cloud
spring-cloud-starter-alibaba-sentinel


com.alibaba.csp
sentinel-datasource-nacos
1.8.6


com.alibaba.cloud
spring-cloud-alibaba-sentinel-gateway

4. 异常如下所示：
java.lang.NullPointerException: value
at java.base/java.util.Objects.requireNonNull(Objects.java:246) ~[na:na]
at reactor.util.context.Context2.put(Context2.java:46) ~[reactor-core-3.4.24.jar:3.4.24]
at com.alibaba.csp.sentinel.adapter.reactor.SentinelReactorSubscriber.currentContext(SentinelReactorSubscriber.java:70) ~[sentinel-reactor-adapter-1.8.6.jar:na]
at com.alibaba.csp.sentinel.adapter.reactor.SentinelReactorSubscriber.currentContext(SentinelReactorSubscriber.java:69) ~[sentinel-reactor-adapter-1.8.6.jar:na]
at reactor.core.publisher.FluxPeek$PeekSubscriber.currentContext(FluxPeek.java:120) ~[reactor-core-3.4.24.jar:3.4.24]
at reactor.core.publisher.InnerOperator.currentContext(InnerOperator.java:33) ~[reactor-core-3.4.24.jar:3.4.24]
at reactor.core.publisher.InnerOperator.currentContext(InnerOperator.java:33) ~[reactor-core-3.4.24.jar:3.4.24]
at reactor.core.publisher.InnerOperator.currentContext(InnerOperator.java:33) ~[reactor-core-3.4.24.jar:3.4.24]

Start by reproducing the reported setup with Spring Cloud Gateway 3.1.4, Sentinel 1.8.6, and Reactor 3.4.24, then read sentinel-reactor-adapter's SentinelReactorSubscriber.java around currentContext at line 70. Trace why the value passed to Reactor Context2.put is null; done means the gateway no longer raises this NullPointerException in the reported setup.

Written by the indexing model from the issue text.