input: Write a function to parse an XML document and return a list of tag objects. Each tag object should contain the id of the tag and a list of values of its subchildren. If the given XML document is empty, return an empty list.
  assert parse_xml('<xml><root><child1 id="1"><subchild1 value="a"></subchild1><subchild2 value="b"></subchild2></child1><child2 id="2"><subchild3 value="c"></subchild3><subchild4 value="d"></subchild4></child2></root></xml>') == "Expected Output"
  assert parse_xml('<xml></xml>') == []
  assert parse_xml('<xml><root><child1 id="1"><subchild1 value="a"></subchild1></child1><child2 id="2"><subchild3 value="c"></subchild3></child2></root></xml>') == "Expected Output"
  assert parse_xml('') == []
output: 
  def parse_xml(xml_str):
      if not xml_str:
          return []
      
      from xml.etree import ElementTree as ET
      
      root = ET.fromstring(xml_str)
      tags = []
      
      for child in root.findall('.//child1') + root.findall('.//child2'):
          tag_id = child.get('id')
          values = [subchild.get('value') for subchild in child.findall('.//subchild1') + child.findall('.//subchild2') + child.findall('.//subchild3') + child.findall('.//subchild4')]
          tags.append({'id': tag_id, 'values': values})
      
      return tags
  