input: Write a function called `create_xml_document` that constructs an XML document using Python from a list of items. The function should take one argument, which is a list of strings, and return the XML document as a string. If the input list is empty, the function should return an empty string.
  
  assert create_xml_document(['Item1', 'Item2', 'Item3'])=='''<?xml version="1.0" ?>
  <data>
      <item>Item1</item>
      <item>Item2</item>
      <item>Item3</item>
  </data>'''
  assert create_xml_document([])==''
output: def create_xml_document(items):
      if not items:
          return ''
      xml_content = '<?xml version="1.0" ?>\n<data>\n'
      for item in items:
          xml_content += f'    <item>{item}</item>\n'
      xml_content += '</data>'
      return xml_content