- [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.

public static final char QUESTION_MASK_CHAR = '?';
    public static final String QUESTION_MASK = valueOf(QUESTION_MASK_CHAR);
    public static final char SLASH_CHAR = '/';
    public static final String SLASH = valueOf(SLASH_CHAR);
    public static final char HYPHEN_CHAR = '-';
    public static final String HYPHEN = valueOf(HYPHEN_CHAR);
    private StringUtils() {
    }
    /**
     * Gets a CharSequence length or {@code 0} if the CharSequence is
     * {@code null}.
     *
     * @param cs a CharSequence or {@code null}
     * @return CharSequence length or {@code 0} if the CharSequence is
     * {@code null}.
     */
    public static int length(final CharSequence cs) {
        return cs == null ? 0 : cs.length();
    }
    /**
     * <p>Repeat a String {@code repeat} times to form a
     * new String.</p>
     *
     * <pre>
     * StringUtils.repeat(null, 2) = null
     * StringUtils.repeat("", 0)   = ""
     * StringUtils.repeat("", 2)   = ""
     * StringUtils.repeat("a", 3)  = "aaa"
     * StringUtils.repeat("ab", 2) = "abab"
     * StringUtils.repeat("a", -2) = ""