Bug fix broken assert

depicts/model.py

@@ -90,7 +90,7 @@ class Item(Base):
     @property
     def artist(self) -> str | None:
         v = wikibase.first_datavalue(typing.cast(Entity, self.entity), "P170")
-        assert isinstance(v, dict)
+        assert v is None or isinstance(v, dict)
         return v["id"] if v else None
 
     @property