Ignore properties without any reference

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43575 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Core/StandardProperty.py

@@ -128,7 +128,9 @@ class StandardProperty(IdAsReferenceMixin('_property'), XMLObject):
     """
     property_default_value = self._getExpressionFromString(self.getPropertyDefault())
 
-    return {'id': self.getReference(),
+    reference = self.getReference()
+    if reference:
+      return {'id': reference,
               'description': self.getDescription(),
               'type': self.getElementaryType(),
               'storage_id': self.getStorageId(),