Fix sensors/history: default area to treatment when None

backend/api/routes/sensors.py

@@ -42,6 +42,6 @@ async def sensors_history(
 ):
     return hub.vine_sensors.get_history(
         device_type=type.value,
-        area=area.value if area else None,
+        area=area.value if area else "treatment",
         hours_back=hours,
     )