[UR][Offload] Don't try to getEventInfo on a null event (#20140)

Null events are created by the UR Offload adapter to represent tasks
which are already complete. As they have no liboffload event associated
with them, don't try to call out to liboffload during `olEventGetInfo`.

Author: RossBrunton

unified-runtime/source/adapters/offload/event.cpp

@@ -32,6 +32,10 @@ UR_APIEXPORT ur_result_t UR_APICALL urEventGetInfo(ur_event_handle_t hEvent,
   case UR_EVENT_INFO_REFERENCE_COUNT:
     return ReturnValue(hEvent->RefCount.load());
   case UR_EVENT_INFO_COMMAND_EXECUTION_STATUS: {
+    if (!hEvent->OffloadEvent) {
+      return ReturnValue(UR_EVENT_STATUS_COMPLETE);
+    }
+
     bool Complete;
     OL_RETURN_ON_ERR(olGetEventInfo(hEvent->OffloadEvent,
                                     OL_EVENT_INFO_IS_COMPLETE, sizeof(Complete),