input: Write a function named `construct_xml` that constructs an XML document using Python based on a list of items and writes it to a file named 'data.xml'. The function should take a list of strings as its parameter. If the input list is empty, it should still generate an 'data' element without any 'item' sub-elements. The generated XML document should contain a root element named 'data', with each item from the input list represented as a 'item' sub-element inside 'data'.
  
  assert construct_xml(['Item1', 'Item2', 'Item3'])==None
  assert construct_xml([])==None
output: def construct_xml(items):
      root = ET.Element("data")
      for item in items:
          ET.SubElement(root, "item").text = item
      tree = ET.ElementTree(root)
      tree.write("data.xml", encoding="utf-8", xml_declaration=True)