[NativeCPU] Handle null phEvent.

The documentation for urEnqueueKernelLaunch marks phEvent as optional,
so make sure we only set it if provided.

Author: hvdijk

source/adapters/native_cpu/enqueue.cpp

@@ -244,15 +244,17 @@ UR_APIEXPORT ur_result_t UR_APICALL urEnqueueKernelLaunch(
 #endif // NATIVECPU_USE_OCK
   event->set_futures(futures);
 
-  *phEvent = event;
+  if (phEvent) {
+    *phEvent = event;
+  }
   event->set_callback([hKernel, event]() {
     event->tick_end();
     // TODO: avoid calling clear() here.
     hKernel->_localArgInfo.clear();
   });
 
   if (hQueue->isInOrder()) {
-    urEventWait(1, phEvent);
+    urEventWait(1, &event);
   }
 
   return UR_RESULT_SUCCESS;