- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Steps to reproduce this issue

1. 提交PR，Travis CI build failed，原因为单元测试不通过
![image](https://user-images.githubusercontent.com/33019873/80856713-bcd40180-8c7e-11ea-8a26-d50021587e85.png)
2. 单元测试要求直接报异常
![image](https://user-images.githubusercontent.com/33019873/80856717-d412ef00-8c7e-11ea-88a3-620b86ddecbe.png)
3. 5月1号解决[#5916]时，将默认方法修改了，现在不会报异常出来
![image](https://user-images.githubusercontent.com/33019873/80856741-0de3f580-8c7f-11ea-8bc8-c527e78d9e04.png)

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

Travis CI build 可以通过

private AbstractDynamicConfiguration configuration;
    @BeforeEach
    public void init() {
        configuration = new AbstractDynamicConfiguration() {
            @Override
            protected String doGetConfig(String key, String group) throws Exception {
                return null;
            }
            @Override
            protected void doClose() throws Exception {
            }
        };
    }
    @Test
    public void testConstants() {
        assertEquals("dubbo.config-center.", PARAM_NAME_PREFIX);
        assertEquals("dubbo.config-center.workers", DEFAULT_THREAD_POOL_PREFIX);
        assertEquals("dubbo.config-center.thread-pool.prefix", THREAD_POOL_PREFIX_PARAM_NAME);
        assertEquals("dubbo.config-center.thread-pool.size", THREAD_POOL_SIZE_PARAM_NAME);
        assertEquals("dubbo.config-center.thread-pool.keep-alive-time", THREAD_POOL_KEEP_ALIVE_TIME_PARAM_NAME);
        assertEquals(1, DEFAULT_THREAD_POOL_SIZE);
        assertEquals(60 * 1000, DEFAULT_THREAD_POOL_KEEP_ALIVE_TIME);
    }
    @Test
    public void testConstructor() {
        URL url = URL.valueOf("default://")