fix: stringify must always return a string

Author: dnlbauer

rocrate/model/preview.py

@@ -70,7 +70,7 @@ def stringify(a):
                 if a._jsonld and a._jsonld['name']:
                     return a._jsonld['name']
                 else:
-                    return a
+                    return str(a)
 
         @template_function
         def is_object_list(a):