publications: Fix sort_order bug for very first publication

mpicms/publications/models.py

@@ -14,7 +14,8 @@
 class LatestFirstOrderable(Orderable):
     def save(self, *args, **kwargs):
         if self.pk is None:
-            self.sort_order = self.__class__.objects.aggregate(models.Min('sort_order'))['sort_order__min'] - 1
+            min = self.__class__.objects.aggregate(models.Min('sort_order'))['sort_order__min']
+            self.sort_order = min - 1 if min is not None else 0
         super(Orderable, self).save(*args, **kwargs)
 
     class Meta: