don't crash on calls missing a next_url info

src/authentic2_gnm/templatetags/gnm.py

@@ -35,6 +35,8 @@ def is_coming_for(request):
                     '{urn:oasis:names:tc:SAML:2.0:protocol}Extensions/{https://www.entrouvert.com/}next_url')[0].text
         except (KeyError, IndexError):
             return 'unknown'
+    if not next_url:
+        return 'unknown'
     target_path = urlparse.urlparse(next_url).path
     for prefix in ('manage', 'admin', 'backoffice'):
         if target_path.startswith('/%s/' % prefix):