Fix new -Wnull-conversion violations detected by a newer clang.

third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc

Index: third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc
diff --git a/third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc b/third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc
index 869763f9262ebc8098cd8d2819a61aab38da95c7..bf868dc30573a6fd87ae71d52335eb85404c94c9 100644
--- a/third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc
+++ b/third_party/sudden_motion_sensor/sudden_motion_sensor_mac.cc
@@ -289,7 +289,7 @@ bool SuddenMotionSensor::Init() {
   size_t local_model_size = sizeof(local_model);
   int params[2] = { CTL_HW, HW_MODEL };
   if (sysctl(params, 2, local_model, &local_model_size, NULL, 0) != 0)
-    return NULL;
+    return false;
 
   const SensorDescriptor* sensor_candidate = NULL;