fix(app); change the NULL to None

app.py

@@ -85,8 +85,8 @@ def predict_machine():
             timestamp=sensor.get("timestamp"),
             risk_score=binary_result.get("risk_score"),
             failure_predicted=binary_result.get("failure_predicted"),
-            failure_type=NULL,
-            predicted_failure_time=NULL,
+            failure_type=None,
+            predicted_failure_time=None,
             confidence=binary_result.get("confidence")
         )
         return binary_result