Make the component_unit predicate work with non seda entities

In the component_unit predicate, we handle the case where
_seda_container_from_context returns None by returning 0.
This can be the case when the predicate is called with a non seda
entity.

cubicweb_seda/entities/__init__.py

@@ -97,7 +97,9 @@ def component_unit(cls, req, rset=None, entity=None, **kwargs):
     container root is not a SEDAArchiveTransfer but a SEDAArchiveUnit).
     """
     entity = _seda_container_from_context(rset, entity, **kwargs)
-    return 1 if entity.cw_etype == 'SEDAArchiveUnit' else 0
+    if entity and entity.cw_etype == 'SEDAArchiveUnit':
+        return 1
+    return 0
 
 
 @objectify_predicate