[balsa] Non-fast-forward update to branch 64-save-empty-attachment

[Peter Bloomfield <peterb src gnome org>]

The branch '64-save-empty-attachment' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  9c8a93f... body: implement libbalsa_message_body_length()
  95743c5... balsa-mime-widget-callbacks: Warn about empty part

Commits added to the branch:

  e0e90fd... body: implement libbalsa_message_body_is_empty()