protected Mono<Void> handleSelectorIfNull(final String pluginName, final ServerWebExchange exchange, final ShenyuPluginChain chain) {
        return chain.execute(exchange);
    }
Handle selector if null mono.

@param pluginName the plugin name
@param exchange the exchange
@param chain the chain
@return the mono
shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/AbstractShenyuPlugin.java