fix: fix PolarisRouterServiceInstanceListSupplier npe with reactive feign (#925)

spring-cloud-starter-tencent-polaris-router/src/main/java/com/tencent/cloud/polaris/router/PolarisRouterServiceInstanceListSupplier.java

@@ -98,6 +98,7 @@ public class PolarisRouterServiceInstanceListSupplier extends DelegatingServiceI
 		PolarisRouterContext routerContext = null;
 
 		DefaultRequestContext requestContext = (DefaultRequestContext) request.getContext();
+		if (requestContext != null) {
 			if (requestContext instanceof RequestDataContext) {
 				routerContext = buildRouterContext(((RequestDataContext) requestContext).getClientRequest().getHeaders());
 			}
@@ -105,6 +106,7 @@ public class PolarisRouterServiceInstanceListSupplier extends DelegatingServiceI
 				routerContext = buildRouterContext(((PolarisLoadBalancerRequest<?>) requestContext.getClientRequest()).getRequest()
 						.getHeaders());
 			}
+		}
 
 		if (routerContext == null) {
 			// return all servers if router context is null.