input: Write a function to create an XML document in Python using the provided list of items. The function should take a list of strings as input and generate an XML structure where each item in the list is a sub-element of the root element 'data'. The function should then write the XML to a file named 'data.xml'. If the input list is empty, no XML file should be created, and the function should return an empty string.
  
  import xml.etree.ElementTree as ET
  
  def create_xml_document(items):
      if not items:
          return ""
  
      # 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')
      return ""
  
  assert create_xml_document(['Item1', 'Item2', 'Item3'])==''
  assert create_xml_document([])==''
output: def create_xml_document(items):
      if not items:
          return ""
  
      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')
      return ""