fix error 500 where the rest framework tries to str-cast None

src/c3nav/routing/route.py

@@ -98,7 +98,7 @@ class Route:
                     if description is None:
                         description = current_space.leave_descriptions.get(next_space.pk, None)
                     if description is None:
-                        description = item.space.enter_description
+                        description = item.space.enter_description or None
                     if description is None:
                         description =  _('Go to %(space_title)s.') % {'space_title': item.space.title}