HAL_PX4: fixed functor comparison with NULL

libraries/AP_HAL_PX4/Scheduler.cpp

@@ -255,7 +255,7 @@ void PX4Scheduler::_run_timers(bool called_from_timer_thread)
     if (!_timer_suspended) {
         // now call the timer based drivers
         for (int i = 0; i < _num_timer_procs; i++) {
-            if (_timer_proc[i] != NULL) {
+            if (_timer_proc[i]) {
                 _timer_proc[i]();
             }
         }
@@ -315,7 +315,7 @@ void PX4Scheduler::_run_io(void)
     if (!_timer_suspended) {
         // now call the IO based drivers
         for (int i = 0; i < _num_io_procs; i++) {
-            if (_io_proc[i] != NULL) {
+            if (_io_proc[i]) {
                 _io_proc[i]();
             }
         }