String result = StringUtil.join(",", null, null, null);
        assertEquals("null,null,null", result);
    }

    @Test
    @DisplayName("Should handle empty string elements")
    void testJoinWithEmptyStringElements() {
        String result = StringUtil.join(",", "", "middle", "");
        // First element is empty string (sb becomes "")
        // Second: sb.length() is 0, no delimiter, append "middle" (sb becomes "middle")