add validation for sensor data and fix tests

internal/domains/sensors/domain.go

@@ -38,12 +38,36 @@ func (s *Sensor) Validate() error {
 	return nil
 }
 
+func (d *SensorData) Validate() error {
+	if d.SensorID == "" {
+		return ErrInvalidSensorIdentifier
+	}
+
+	if d.Value == nil {
+		return ErrMissingValue
+	}
+
+	if d.Timestamp == nil {
+		now := time.Now()
+		d.Timestamp = &now
+	}
+
+	return nil
+}
+
+// TODO: implement this in service layer for alerts
 func (d *SensorData) IsOutOfRangeAbove(sensor Sensor) bool {
-	return d.Value > *sensor.ThresholdAbove
+	if d.Value == nil || sensor.ThresholdAbove == nil {
+		return false
+	}
+	return *d.Value > *sensor.ThresholdAbove
 }
 
 func (d *SensorData) IsOutOfRangeBelow(sensor Sensor) bool {
-	return d.Value < *sensor.ThresholdBelow
+	if d.Value == nil || sensor.ThresholdBelow == nil {
+		return false
+	}
+	return *d.Value < *sensor.ThresholdBelow
 }
 
 func (r *SensorRequest) Validate() error {