fix: initialize Activity with Commit post-init

closes #834

Author: rokroskar

renku/core/models/provenance/activities.py

@@ -278,6 +278,7 @@ def _parents(node):
 
     def __attrs_post_init__(self):
         """Sets ``generated`` default value if it's not set already."""
+        super().__attrs_post_init__()
         if not self.generated:
             self.generated = self.default_generated()