Return an empty string if the URL is None. This happens usually when an image is not uploaded yet.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@26632 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Form/ImageField.py

@@ -75,6 +75,8 @@ class ImageFieldWidget(Widget.TextWidget):
         """Render image field as a link to the image
         """
         # Url is already defined in value
+        if value is None:
+          return ''
         image = value
         alt = field.get_value('description') or \
               field.get_value('title')