- [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.

assertThat(StringUtils.isContains((String[]) null, null), is(false));
        assertTrue(StringUtils.isContains("abc", 'a'));
        assertFalse(StringUtils.isContains("abc", 'd'));
        assertFalse(StringUtils.isContains("", 'a'));
        assertFalse(StringUtils.isContains(null, 'a'));
        assertTrue(StringUtils.isNotContains("abc", 'd'));
        assertFalse(StringUtils.isNotContains("abc", 'a'));
        assertTrue(StringUtils.isNotContains("", 'a'));
        assertTrue(StringUtils.isNotContains(null, 'a'));
    }
    @Test
    public void testIsNumeric() throws Exception {
        assertThat(StringUtils.isNumeric("123", false), is(true));
        assertThat(StringUtils.isNumeric("1a3", false), is(false));
        assertThat(StringUtils.isNumeric(null, false), is(false));
        assertThat(StringUtils.isNumeric("0", true), is(true));
        assertThat(StringUtils.isNumeric("0.1", true), is(true));
        assertThat(StringUtils.isNumeric("DUBBO", true), is(false));
        assertThat(StringUtils.isNumeric("", true), is(false));
        assertThat(StringUtils.isNumeric(" ", true), is(false));
        assertThat(StringUtils.isNumeric("   ", true), is(false));
        assertThat(StringUtils.isNumeric("123.3.3", true), is(false));
        assertThat(StringUtils.isNumeric("123.", true), is(true));
        assertThat(StringUtils.isNumeric(".123", true), is(true));
        assertThat(StringUtils.isNumeric("..123", true), is(false));
    }
    @Test
    public void testJoinCollectionString() throws Exception {
        List<String> list = new ArrayList<String>();
        assertEquals("", StringUtils.join(list, ","));