- [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.

return buf.toString();
    }
    public static String trim(String str) {
        return str == null ? null : str.trim();
    }
    public static String toURLKey(String key) {
        return key.toLowerCase().replaceAll(SEPARATOR_REGEX, HIDE_KEY_PREFIX);
    }
    public static String toOSStyleKey(String key) {
        key = key.toUpperCase().replaceAll(DOT_REGEX, UNDERLINE_SEPARATOR);
        if (!key.startsWith("DUBBO_")) {
            key = "DUBBO_" + key;
        }
        return key;
    }
    public static boolean isAllUpperCase(String str) {
        if (str != null && !isEmpty(str)) {
            int sz = str.length();
            for (int i = 0; i < sz; ++i) {
                if (!Character.isUpperCase(str.charAt(i))) {
                    return false;
                }
            }
            return true;
        } else {
            return false;
        }
    }
    public static String[] delimitedListToStringArray(String str, String delimiter) {
        return delimitedListToStringArray(str, delimiter, (String) null);