当在xpath中没有找到任何东西时，如何返回NA？

import lxml.etree as ET

def find_element(xpath_expression, xml_content):
    root = ET.fromstring(xml_content)
    elements = root.xpath(xpath_expression)
    
    if len(elements) == 0:
        return "NA"
    
    return elements[0].text

# 示例用法
xml = """
<root>
    <element>Value 1</element>
    <element>Value 2</element>
</root>
"""

result = find_element("//element[3]", xml)
print(result)  # 输出: NA