erp5_web: make sure link has value

bt5/erp5_web/DocumentTemplateItem/portal_components/document.erp5.ReplaceRelativeUrlTextDocument.py

@@ -35,6 +35,7 @@ class ReplaceRelativeUrlTextDocument(TextDocument, object):
           # By conversion, first letter of reference should be upper case
           # To be more sure of what to replace
           # erp5-Howto
+          if new_link:
             if new_link[0].isupper() or new_link.startswith('erp5-') or new_link.startswith('user-') or \
                new_link.startswith('slapos-') or new_link.startswith('vifib-') or new_link.startswith('osoe-') or \
                new_link.startswith('developer-') or new_link.startswith('slapOS'):