HDFS-16704. Datanode return empty response instead of NPE for GetVolumeInfo during restarting (#4661). Contributed by ZanderXu.

Signed-off-by: Ayush Saxena <ayushsaxena@apache.org>
Signed-off-by: He Xiaoqiao <hexiaoqiao@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java

@@ -3628,7 +3628,10 @@ public class DataNode extends ReconfigurableBase
    */
   @Override // DataNodeMXBean
   public String getVolumeInfo() {
-    Preconditions.checkNotNull(data, "Storage not yet initialized");
+    if (data == null) {
+      LOG.debug("Storage not yet initialized.");
+      return "";
+    }
     return JSON.toString(data.getVolumeInfoMap());
   }