}

    public void test_stripInvalidXMLCharacters_null() {
        // Test null input
        assertNull(XmlUtil.stripInvalidXMLCharacters(null));
    }

    public void test_stripInvalidXMLCharacters_empty() {
        // Test empty string
        assertEquals("", XmlUtil.stripInvalidXMLCharacters(""));
    }

    public void test_stripInvalidXMLCharacters_valid() {
        // Test valid XML characters