[Stats] fix stats shutdown crash if opencensus exporter not initialized (#9727)

src/ray/stats/stats.h

@@ -103,6 +103,10 @@ static inline void Init(const TagsType &global_tags, const int metrics_agent_por
 static inline void Shutdown() {
   // TODO(sang): Harvest thread is not currently cleaned up.
   absl::MutexLock lock(&stats_mutex);
+  if (!StatsConfig::instance().IsInitialized()) {
+    // Return if stats had never been initialized.
+    return;
+  }
   metrics_io_service_pool->Stop();
   opencensus::stats::StatsExporter::Shutdown();
   metrics_io_service_pool = nullptr;