- [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.

assertTrue(StringUtils.isAnyEmpty("abc", ""));
        assertFalse(StringUtils.isAnyEmpty("abc", " "));
    }
    @Test
    public void testIsNotEmpty() throws Exception {
        assertFalse(StringUtils.isNotEmpty(null));
        assertFalse(StringUtils.isNotEmpty(""));
        assertTrue(StringUtils.isNotEmpty("abc"));
    }
    @Test
    public void testIsEquals() throws Exception {
        assertTrue(StringUtils.isEquals(null, null));
        assertFalse(StringUtils.isEquals(null, ""));
        assertTrue(StringUtils.isEquals("abc", "abc"));
        assertFalse(StringUtils.isEquals("abc", "ABC"));
    }
    @Test
    public void testIsInteger() throws Exception {
        assertFalse(StringUtils.isInteger(null));
        assertFalse(StringUtils.isInteger(""));
        assertTrue(StringUtils.isInteger("123"));
    }
    @Test
    public void testParseInteger() throws Exception {
        assertThat(StringUtils.parseInteger(null), equalTo(0));
        assertThat(StringUtils.parseInteger("123"), equalTo(123));
    }
    @Test
    public void testIsJavaIdentifier() throws Exception {
        assertThat(StringUtils.isJavaIdentifier(""), is(false));