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

### Environment

* Dubbo version: 2.7.4
* Operating System version: Darwin Kernel Version 18.7.0
* Java version: 1.8.0_181-b13)

### Steps to reproduce this issue

1. invoke `StringUtils.split("d,1,2,4", 'a')`
2. invoke `"d,1,2,4".split("a")`

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

### Expected Result

`StringUtils.split("d,1,2,4", 'a')` should return `["d,1,2,4"]` just like `"d,1,2,4".split("a")`.

### Actual Result

empty array.

@Test
    public void testLength() throws Exception {
        assertThat(StringUtils.length(null), equalTo(0));
        assertThat(StringUtils.length("abc"), equalTo(3));
    }
    @Test
    public void testRepeat() throws Exception {
        assertThat(StringUtils.repeat(null, 2), nullValue());
        assertThat(StringUtils.repeat("", 0), equalTo(""));
        assertThat(StringUtils.repeat("", 2), equalTo(""));
        assertThat(StringUtils.repeat("a", 3), equalTo("aaa"));
        assertThat(StringUtils.repeat("ab", 2), equalTo("abab"));
        assertThat(StringUtils.repeat("a", -2), equalTo(""));
        assertThat(StringUtils.repeat(null, null, 2), nullValue());
        assertThat(StringUtils.repeat(null, "x", 2), nullValue());
        assertThat(StringUtils.repeat("", null, 0), equalTo(""));
        assertThat(StringUtils.repeat("", "", 2), equalTo(""));
        assertThat(StringUtils.repeat("", "x", 3), equalTo("xx"));
        assertThat(StringUtils.repeat("?", ", ", 3), equalTo("?, ?, ?"));
        assertThat(StringUtils.repeat('e', 0), equalTo(""));
        assertThat(StringUtils.repeat('e', 3), equalTo("eee"));
    }
    @Test
    public void testStripEnd() throws Exception {
        assertThat(StringUtils.stripEnd(null, "*"), nullValue());
        assertThat(StringUtils.stripEnd("", null), equalTo(""));
        assertThat(StringUtils.stripEnd("abc", ""), equalTo("abc"));