When saving an attachment headerFormatProperties can throw an exception when its input is None #324

msg = extract_msg.Message(file_path)
for attachment in msg.attachments:
  attachment.save(customPath=output_path)

  File "/home/user/project/process_msg_emails.py", line 46, in <module>
    extract_msg_file(file_path, output_path)
  File "/home/user/project/chronodata/emails/process_msg_files.py", line 30, in extract_msg_file
    attachment.save(customPath=output_path)
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/attachment.py", line 241, in save
    self.saveEmbededMessage(**kwargs)
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/attachment.py", line 254, in saveEmbededMessage
    self.data.save(**kwargs)
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/message_base.py", line 902, in save
    f.write(self.getSaveBody(**kwargs))
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/message_base.py", line 300, in getSaveBody
    header = self.getInjectableHeader(prefix, joinStr, suffix, formatter).encode('utf-8')
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/message_base.py", line 227, in getInjectableHeader
    allProps = self.headerFormatProperties
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/contact.py", line 722, in headerFormatProperties
    'Business 2': strListToStr(self.businessTelephone2Number),
  File "/home/user/project/.venv/lib/python3.9/site-packages/extract_msg/contact.py", line 699, in strListToStr
    return ', '.join(inp)
TypeError: can only join an iterable