- [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 String EMPTY_STRING = "";
    public static final int INDEX_NOT_FOUND = -1;
    public static final String[] EMPTY_STRING_ARRAY = new String[0];
    private static final Logger logger = LoggerFactory.getLogger(StringUtils.class);
    private static final Pattern KVP_PATTERN = Pattern.compile("([_.a-zA-Z0-9][-_.a-zA-Z0-9]*)[=](.*)"); 
    private static final Pattern INT_PATTERN = Pattern.compile("^\\d+$");
    private static final Pattern PARAMETERS_PATTERN = Pattern.compile("^\\[((\\s*\\{\\s*[\\w_\\-\\.]+\\s*:\\s*.+?\\s*\\}\\s*,?\\s*)+)\\s*\\]$");
    private static final Pattern PAIR_PARAMETERS_PATTERN = Pattern.compile("^\\{\\s*([\\w-_\\.]+)\\s*:\\s*(.+)\\s*\\}$");
    private static final int PAD_LIMIT = 8192;
    /**
     * @since 2.7.5
     */
    public static final char EQUAL_CHAR = '=';
    public static final String EQUAL = valueOf(EQUAL_CHAR);
    public static final char AND_CHAR = '&';
    public static final String AND = valueOf(AND_CHAR);
    public static final char SEMICOLON_CHAR = ';';
    public static final String SEMICOLON = valueOf(SEMICOLON_CHAR);