private Pair<Boolean, RuleData> matchRule(final ServerWebExchange exchange, final Collection<RuleData> rules) {
        List<RuleData> filterRuleData = rules.stream()
                .filter(rule -> filterRule(rule, exchange))
                .distinct()
                .collect(Collectors.toList());
        if (filterRuleData.size() > 1) {
            return Pair.of(Boolean.FALSE, manyMatchRule(filterRuleData));
        } else {
            return Pair.of(Boolean.TRUE, filterRuleData.stream().findFirst().orElse(null));
        }
    }
Handle rule 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