如何在不丢弃不存在值的节点的情况下从R中的复杂XML中提取值？

<root>
  <person id="1">
    <name>John Doe</name>
    <age>30</age>
    <address>
      <city>New York</city>
      <zip>10001</zip>
    </address>
  </person>
  <person id="2">
    <name>Jane Smith</name>
    <age>25</age>
    <address>
      <city>Los Angeles</city>
      <zip>90001</zip>
    </address>
  </person>
  <person id="3">
    <name>Bob Johnson</name>
    <age>40</age>
  </person>
</root>

library(xml2)

# 读取XML文档
xml_data <- read_xml("<root>...</root>")  # 替换为实际的XML内容

# 提取所有person节点
persons <- xml_find_all(xml_data, ".//person")

# 遍历每个person节点并提取数据
for (person in persons) {
  name <- xml_text(xml_find_first(person, ".//name"))
  age <- xml_text(xml_find_first(person, ".//age"))
  city <- xml_text(xml_find_first(person, ".//address/city"))
  zip <- xml_text(xml_find_first(person, ".//address/zip"))
  
  # 打印结果
  cat("Name:", name, "Age:", age, "City:", city, "Zip:", zip, "\n")
}