Handle empty Link for transitive property

.. in REST API: If we request a transitive property in parameters (e.g.
author.username) and the Link is empty (None), we now return a username
of None and do not fail with a 404.

Author: schlatterbeck

roundup/rest.py

@@ -627,6 +627,12 @@ def format_item(self, node, item_id, props=None, verbose=1):
                     cl = self.db.getclass(cn)
                     nd = cl.getnode(id)
                     id = v = getattr(nd, p)
+                    # Handle transitive properties where something on
+                    # the road is None (empty Link property)
+                    if id is None :
+                        prop = None
+                        ok = True
+                        break
                     prop = cl.getprops(protected=True)[p]
                     cn = getattr(prop, 'classname', None)
                 else: