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

    @Test
    @DisplayName("Should handle whitespace delimiter")
    void testJoinWithWhitespaceDelimiter() {
        String result = StringUtil.join("   ", "spaced", "out");
        assertEquals("spaced   out", result);
    }

    @Test
    @DisplayName("Should handle elements containing delimiter")