[ui] Handle case where there is no concept matching the constraint

This triggered a test failure.

views/dataobject.py

@@ -102,7 +102,8 @@ class ContainedSearchForRelatedEntitiesView(rwdg.SearchForRelatedEntitiesView):
             if lang in languages:
                 break
         else:
-            assert languages, 'no preferred label'
+            if not languages:
+                return self._cw.empty_rset()
             lang = languages.pop()
         # now build the query to prefetch/sort on the preferred label in the proper language
         return self._cw.execute(