// Third: sb.length() > 0, add delimiter + "" (sb becomes "middle,")
        assertEquals("middle,", result);
    }

    @Test
    @DisplayName("Should handle mix of empty and null elements")
    void testJoinWithMixedEmptyAndNull() {
        String result = StringUtil.join("-", "", null, "value", "", null);
        // First: "" (sb becomes "")
        // Second: sb.length() is 0, no delimiter, append null (sb becomes "null")