input: Write a function to construct an XML document using Python with the given list of items. The items should be nested under a root element named 'data', and each item should be represented as a sub-element named 'item'. The function should save the resulting XML tree to a file named 'data.xml'.
  
  def construct_xml_document(items):
      import xml.etree.ElementTree as ET
      
      # creating the root element
      data = ET.Element('data')
      
      # loop over items
      for item in items:
          # creating sub elements
          item_element = ET.SubElement(data, 'item')
          
          # assign the value for the sub elements
          item_element.text = item
      
      # write the XML Tree
      tree = ET.ElementTree(data)
      tree.write('data.xml')
  
  # Test cases
  assert construct_xml_document(['Item1', 'Item2', 'Item3']) == None
  assert construct_xml_document(['ProductA', 'ProductB']) == None
  assert construct_xml_document(['Book1', 'Book2', 'Book3', 'Book4']) == None
  assert construct_xml_document([]) == None
output: def construct_xml_document(items):
      import xml.etree.ElementTree as ET
      
      data = ET.Element('data')
      
      for item in items:
          item_element = ET.SubElement(data, 'item')
          item_element.text = item
      
      tree = ET.ElementTree(data)
      tree.write('data.xml')