Fix NPE

### What changes are proposed in this pull request?

Please outline the changes and how this PR fixes the issue.

### Why are the changes needed?

Please clarify why the changes are needed. For instance,
  1. If you propose a new API, clarify the use case for a new API.
  2. If you fix a bug, describe the bug.

### Does this PR introduce any user facing changes?

Please list the user-facing changes introduced by your change, including
  1. change in user-facing APIs
  2. addition or removal of property keys
  3. webui

			pr-link: #18391
			change-id: cid-24fcdc50fc0b6560f77c0f2f31ebadd78000bac8

dora/core/server/master/src/main/java/alluxio/master/job/LoadSubTask.java

@@ -106,10 +106,11 @@ public static LoadSubTask from(LoadFailure loadFailure, long virtualBlockSize) {
 
   @Override
   public String toString() {
+    WorkerInfo workerInfo = getWorkerInfo();
     return MoreObjects.toStringHelper(this)
         .add("UfsPath", getUfsPath())
-        .add("ShardingKey", mHashKey.asString())
-        .add("Worker", getWorkerInfo().getAddress().toString())
+        .add("ShardingKey", mHashKey == null ? null : mHashKey.asString())
+        .add("Worker", workerInfo == null ? null : workerInfo.getAddress().toString())
         .toString();
   }
 }