Fix return type of soft delete methods

I don't know whether returning "nothing was deleted" is the best
option, but at least the returned value now has the correct type.

jazzband#541

model_utils/managers.py

@@ -250,6 +250,7 @@ def delete(self):
         field to True)
         """
         self.update(is_removed=True)
+        return (0, {})  # no objects were actually deleted
 
 
 class SoftDeletableQuerySet(SoftDeletableQuerySetMixin, QuerySet):

model_utils/models.py

@@ -156,6 +156,7 @@ def delete(self, using=None, soft=True, *args, **kwargs):
         if soft:
             self.is_removed = True
             self.save(using=using)
+            return (0, {})  # no objects were actually deleted
         else:
             return super().delete(using=using, *args, **kwargs)