Fixed the reloadRules method that caused ruleMetaData.getRules() to be null briefly

[hua74ni]

Changes proposed in this pull request:

• Fixed the reloadRules method that caused ruleMetaData.getRules() to be null briefly

---

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• My code follows the code of conduct of this project.
• I have self-reviewed the commit code.
• I have (or in comment I request) added corresponding labels for the pull request.
• I have passed maven check locally : ./mvnw clean install -B -T1C -Dmaven.javadoc.skip -Dmaven.jacoco.skip -e.
• I have made corresponding changes to the documentation.
• I have added corresponding unit tests for my changes.

[hua74ni]

Test cases

@Test
public void assertTest() throws InterruptedException {

    ShardingSphereRuleMetaData ruleMetaData = new ShardingSphereRuleMetaData(Collections.singleton(new ShardingSphereRuleFixture()));

    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("start cycle get ruleMetaData rules...");
            while (true) {
                if (CollectionUtils.isEmpty(ruleMetaData.getRules())) {
                    System.out.println("ruleMetaData rules is null!!!!");
                }
            }
        }
    });
    thread.setDaemon(true);
    thread.start();

    Thread.sleep(1000);

    boolean flag = false;
    System.out.println("start replace all ruleMetaData rules...");
    if (flag) {
        ((CopyOnWriteArrayList) ruleMetaData.getRules()).replaceAll(shardingSphereRule -> {
            return new ShardingSphereRuleFixture();
        });
    } else {
        ShardingSphereRuleMetaData newRuleMetaData = new ShardingSphereRuleMetaData(Collections.singleton(new ShardingSphereRuleFixture()));
        ruleMetaData.getRules().clear();
        ruleMetaData.getRules().addAll(newRuleMetaData.getRules());
    }

    System.out.println("finished replace all ruleMetaData rules...");

    Thread.sleep(3000);
}

[hua74ni]

@zhaojinchao95 Help review or make suggestions