fix polymorphic_on check

sqlmodel/_compat.py

@@ -294,6 +294,7 @@ if IS_PYDANTIC_V2:
         # Override polymorphic_on default value
         mapper = inspect(cls)
         polymorphic_on = mapper.polymorphic_on
+        if polymorphic_on:
             polymorphic_property = mapper.get_property_by_column(polymorphic_on)
             field_info = cls.model_fields.get(polymorphic_property.key)
             if field_info:
@@ -302,7 +303,9 @@ if IS_PYDANTIC_V2:
                 # set the polymorphic_on by default
                 if mapper.inherits or v is None:
                     setattr(
-                    self_instance, polymorphic_property.key, mapper.polymorphic_identity
+                        self_instance,
+                        polymorphic_property.key,
+                        mapper.polymorphic_identity,
                     )
         return self_instance