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

    @Test
    @DisplayName("Should handle all null elements")
    void testJoinWithAllNullElements() {
        String result = StringUtil.join(",", null, null, null);
        assertEquals("null,null,null", result);
    }

    @Test
    @DisplayName("Should handle empty string elements")