Fix NPE in IndexShard flushStats

Note we cannot use withEngineOrNull because the method can block
and cannot be called from cluster state update threads.

Fixes #124164

Author: kingherc

server/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -1366,11 +1366,12 @@ public RefreshStats refreshStats() {
     }
 
     public FlushStats flushStats() {
+        final Engine engine = getEngineOrNull();
         return new FlushStats(
             flushMetric.count(),
             periodicFlushMetric.count(),
             TimeUnit.NANOSECONDS.toMillis(flushMetric.sum()),
-            getEngineOrNull() != null ? getEngineOrNull().getTotalFlushTimeExcludingWaitingOnLockInMillis() : 0L
+            engine != null ? engine.getTotalFlushTimeExcludingWaitingOnLockInMillis() : 0L
         );
     }