Make subject an optional property of sms to make it compatible with Signal

[DerBeutlin]

I use signal-back to convert signal-backup files to xml files compatible with SMS Backup & Restore. Unfortunately the subject property is missing. Therefore I would like to make the subject property optional, replacing it with the empty string if it's not available.

[novoid]

Hey, I too tested signal-back to be able to use this Memacs module with it. ;-)
Unfortunately, the format seems to have changed to big extend since I first wrote the sms Module. For example, every Signal message with an "attachment" is now of type "mms" which is not supported by the memacs module yet.
Therefore, adding support for more XML elements is on my mid-term todo list.
Glad that there is another person using the same workflow ;-)