- [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.

if (i > 0) {
                sb.append(split);
            }
            sb.append(array[i]);
        }
        return sb.toString();
    }
    /**
     * join string like javascript.
     *
     * @param array String array.
     * @param split split
     * @return String.
     */
    public static String join(String[] array, String split) {
        if (ArrayUtils.isEmpty(array)) {
            return EMPTY_STRING;
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < array.length; i++) {
            if (i > 0) {
                sb.append(split);
            }
            sb.append(array[i]);
        }
        return sb.toString();
    }
    public static String join(Collection<String> coll, String split) {
        if (CollectionUtils.isEmpty(coll)) {
            return EMPTY_STRING;