mixer_module: send a last sample out after all outputs were disabled

This matters for PWM when the last output gets disabled on either FMU or IO
it would just keep on running.

Also when rebooting with a parameters reset or new airframe with no mapped outputs
it would previously keep outputting PWM with the disarmed value of the new airframe
e.g. 1000us which is a safety hazard because servos could break the physical limit of the
model or miscalibrated ESCs spinning motors.

src/lib/mixer_module/mixer_module.cpp

@@ -455,7 +455,8 @@ bool MixingOutput::update()
 		}
 	}
 
-	if (!all_disabled) {
+	// Send output if any function mapped or one last disabling sample
+	if (!all_disabled || !_was_all_disabled) {
 		if (!_armed.armed && !_armed.manual_lockdown) {
 			_actuator_test.overrideValues(outputs, _max_num_outputs);
 		}
@@ -463,6 +464,8 @@ bool MixingOutput::update()
 		limitAndUpdateOutputs(outputs, has_updates);
 	}
 
+	_was_all_disabled = all_disabled;
+
 	return true;
 }