cfix: Create and initialize attribute

Resolves the following traceback, happening when opening a chat containing filetransfers and the filetransfer db table contains data.

Traceback

Traceback (most recent call last):
  File "gajim/gajim/gtk/chat_page.py", line 134, in _on_chat_selected
    self._chat_stack.show_chat(account, jid)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "gajim/gajim/gtk/chat_stack.py", line 213, in show_chat
    self._chat_control.switch_contact(self._current_contact)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "gajim/gajim/gtk/control.py", line 225, in switch_contact
    self._request_history(None, True)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "gajim/gajim/gtk/control.py", line 593, in _request_history
    self._add_messages([row])
    ~~~~~~~~~~~~~~~~~~^^^^^^^
  File "gajim/gajim/gtk/control.py", line 545, in _add_messages
    self._add_db_row(msg)
    ~~~~~~~~~~~~~~~~^^^^^
  File "gajim/gajim/gtk/control.py", line 534, in _add_db_row
    self._scrolled_view.add_jingle_file_transfer(message=message)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "gajim/gajim/gtk/conversation/view.py", line 478, in add_jingle_file_transfer
    jingle_transfer_row = FileTransferJingleRow(
        self._contact.account, self._contact, event=event, message=message
    )
  File "gajim/gajim/gtk/conversation/rows/file_transfer_jingle.py", line 161, in __init__
    self._reconstruct_transfer()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "gajim/gajim/gtk/conversation/rows/file_transfer_jingle.py", line 187, in _reconstruct_transfer
    self._show_file_infos()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "gajim/gajim/gtk/conversation/rows/file_transfer_jingle.py", line 219, in _show_file_infos
    if self._file_props is None:
       ^^^^^^^^^^^^^^^^
AttributeError: 'FileTransferJingleRow' object has no attribute '_file_props'