public static String toString(InputStream input, String encoding) throws IOException {
        if (input == null) {
            return StringUtils.EMPTY;
        }
        return (null == encoding) ? toString(new InputStreamReader(input, Constants.ENCODE))
                : toString(new InputStreamReader(input, encoding));
    }
@Test
    void testToStringWithNull() throws IOException {
        assertEquals("", IoUtils.toString(null, "UTF-8"));
    }