calibration params: avoid using NaN as default

- NaN is not supported by JSON, and leads to parsing failure in QGC.
 - fixes https://github.com/PX4/PX4-Autopilot/issues/18095

src/lib/sensor_calibration/Accelerometer.hpp

@@ -92,6 +92,8 @@ public:
 	void SensorCorrectionsUpdate(bool force = false);
 
 private:
+	static constexpr float TEMPERATURE_INVALID = -1000.f;
+
 	uORB::Subscription _sensor_correction_sub{ORB_ID(sensor_correction)};
 
 	Rotation _rotation_enum{ROTATION_NONE};