MMS: Gracefully handle SIMs that don't provide own number

Currently Chatty prints a warning in case of a SIM that doesn't provide own number:

14:54:52.6194          chatty-mmsd[ 3384]: WARNING: Your SIM or Modem does not support modem manger's number! Please file a bug report

This should rather be handled gracefully, since such SIMs are quite common.

Also:

<kop316>: The issue in chatty, which needs to be fixed somehow, is if chatty doesn't get it's own number, than the MMS will not filter out the number of the own modem, which is confusing to the user. To be honest, I don't really know of a good way to fix that