public static String defaultString(final Object str) {
        return toString(str, EMPTY);
    }
<p>Returns either the passed in String,
or if the String is {@code null}, an empty String ("").</p>

<pre>
StringUtils.defaultString(null)  = ""
StringUtils.defaultString("")    = ""
StringUtils.defaultString("bat") = "bat"
</pre>

@param str the String to check, may be null
@return the passed in String, or the empty String if it
was {@code null}
@see String#valueOf(Object)
core/common/src/main/java/com/alipay/sofa/rpc/common/utils/StringUtils.java