NullMixer - Send NAN instead of 0. NAN is mapped to disarmed in fmu and io. A...

NullMixer - Send NAN instead of 0. NAN is mapped to disarmed in fmu and io. A Null mixer can now be used to set a fixed value: "disarmed" if the system is operational or "failsafe" in failsafe mode

src/lib/mixer/mixer.cpp

@@ -189,7 +189,7 @@ unsigned
 NullMixer::mix(float *outputs, unsigned space)
 {
 	if (space > 0) {
-		*outputs = 0.0f;
+		*outputs = NAN;
 		return 1;
 	}