[FIX] error with empty author_id

__openerp__.py

@@ -1,6 +1,6 @@
 {
     'name' : 'Sentbox',
-    'version' : '1.0.0',
+    'version' : '1.0.1',
     'author' : 'IT-Projects LLC, Ivan Yelizariev',
     'license': 'GPL-3',
     'category' : 'Social Network',

models.py

@@ -6,7 +6,7 @@ class mail_message(models.Model):
     @api.one
     @api.depends('author_id', 'notified_partner_ids')
     def _get_sent(self):
-        self.sent = len(self.notified_partner_ids) > 1 or len(self.notified_partner_ids)==1 and self.notified_partner_ids[0].id != self.author_id.id
+        self.sent = len(self.notified_partner_ids) > 1 or len(self.notified_partner_ids)==1 and self.author_id and self.notified_partner_ids[0].id != self.author_id.id
 
     sent = fields.Boolean('Sent', compute=_get_sent, help='Was message sent to someone', store=True)