allow reading HBA_time_beam for LBA observations

tangostationcontrol/tangostationcontrol/devices/observation_field.py

@@ -220,9 +220,9 @@ class ObservationField(LOFARDevice):
     def HBA_tile_beam_R(self):
         try:
             if self._observation_field_settings.HBA.tile_beam is None:
-                return None
+                return []
         except AttributeError:
-            return None
+            return []
 
         pointing_direction = self._observation_field_settings.HBA.tile_beam
         return [