🔇(backend) fix E010 Django warning logs

Room model uses a default value for its configuration.
However, I used a wrong default value, it should be a callable.

Align the code with Magnify.

src/backend/core/models.py

@@ -296,7 +296,7 @@ class Room(Resource):
 
     configuration = models.JSONField(
         blank=True,
-        default={},
+        default=dict,
         verbose_name=_("Visio room configuration"),
         help_text=_("Values for Visio parameters to configure the room."),
     )