- [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.

* {@code null} if null String input
     * @since 2.5
     */
    public static String repeat(final String str, final String separator, final int repeat) {
        if (str == null || separator == null) {
            return repeat(str, repeat);
        }
        
        final String result = repeat(str + separator, repeat);
        return removeEnd(result, separator);
    }
    /**
     * <p>Removes a substring only if it is at the end of a source string,
     * otherwise returns the source string.</p>
     *
     * <p>A {@code null} source string will return {@code null}.
     * An empty ("") source string will return the empty string.
     * A {@code null} search string will return the source string.</p>
     *
     * <pre>
     * StringUtils.removeEnd(null, *)      = null
     * StringUtils.removeEnd("", *)        = ""
     * StringUtils.removeEnd(*, null)      = *
     * StringUtils.removeEnd("www.domain.com", ".com.")  = "www.domain.com"
     * StringUtils.removeEnd("www.domain.com", ".com")   = "www.domain"
     * StringUtils.removeEnd("www.domain.com", "domain") = "www.domain.com"
     * StringUtils.removeEnd("abc", "")    = "abc"
     * </pre>
     *
     * @param str    the source String to search, may be null