fix comparaison objets nuls

src/agenda_culturel/models.py

@@ -146,6 +146,7 @@ class DuplicatedEvents(models.Model):
 
     def get_item_comparison(self, attr):
         values = [getattr(e, attr) for e in self.get_duplicated()]
+        values = ["" if v is None else v for v in values]
 
         if len(set([str(v) for v in values])) == 1:
             return { "similar": True, "key": attr, "values": values[0] }