Calrify why we don't raise Attribute error here

Co-authored-by: Nicola Soranzo <nicola.soranzo@gmail.com>

lib/galaxy/model/metadata.py

@@ -113,6 +113,7 @@ class MetadataCollection(Mapping):
             return self.spec[name].wrap(self.spec[name].default, object_session(self.parent))
         if name in self.parent._metadata:
             return self.parent._metadata[name]
+        # Instead of raising an AttributeError for non-existing metadata, we return None
         return None
 
     def __setattr__(self, name, value):