HBASE-28598 Fixes NPE for writer object access in AsyncFSWAL#closeWriter (#5907)

Signed-off-by: Duo Zhang <zhangduo@apache.org>
(cherry picked from commit 17ae116)

Author: vineet4008

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AsyncFSWAL.java

@@ -772,8 +772,10 @@ protected void doReplaceWriter(Path oldPath, Path newPath, AsyncWriter nextWrite
   @Override
   protected void doShutdown() throws IOException {
     waitForSafePoint();
-    closeWriter(this.writer, getOldPath());
-    this.writer = null;
+    if (this.writer != null) {
+      closeWriter(this.writer, getOldPath());
+      this.writer = null;
+    }
     closeExecutor.shutdown();
     try {
       if (!closeExecutor.awaitTermination(waitOnShutdownInSeconds, TimeUnit.SECONDS)) {