code: def _resolve_file(file_name):
      
      if not file_name:
          return None
  
      path = os.path.realpath(file_name)
      if os.path.isfile(path):
          return path
  
      return None
docs: Checks if the file exists.
  
      If the file exists, the method returns its absolute path.
      Else, it returns None
  
      :param file_name: The name of the file to check
      :return: An absolute path, or None