Be resilient to a missing 'entry' in urgency (#3479)

Any combination of properties is possible - Taskwarrior should make
the best of the tasks it finds, rather than crashing.

src/Task.cpp

@@ -2319,7 +2319,8 @@ float Task::urgency_due () const
 ////////////////////////////////////////////////////////////////////////////////
 float Task::urgency_age () const
 {
-  assert (has ("entry"));
+  if (!has ("entry"))
+    return 1.0;
 
   Datetime now;
   Datetime entry (get_date ("entry"));