return empty string if proxy field is an empty string

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@5580 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Form/ProxyField.py

@@ -111,6 +111,8 @@ class ProxyWidget(Widget.Widget):
     """
       Display proxy field
     """
+    if type(value) == type('') and value == '':
+      return ''
     form = field.aq_parent
     try:
       proxy_form = getattr(form, field.get_value('form_id'))