### What changes were proposed in this pull request?
Fix NPE if `celeborn.client.spark.fetch.cleanFailedShuffle` is true.
This PR also refine the code for `FailedShuffleCleaner`.
### Why are the changes needed?
`failedShuffleCleaner` is null in executor end.
```
25/07/29 17:58:40 ERROR SparkUncaughtExceptionHandler: Uncaught exception in thread Thread[CoarseGrainedExecutorBackend-stop-executor,5,main]
java.lang.NullPointerException: Cannot invoke "org.apache.celeborn.spark.FailedShuffleCleaner.reset()" because "this.failedShuffleCleaner" is null
at org.apache.spark.shuffle.celeborn.SparkShuffleManager.stop(SparkShuffleManager.java:272) ~[celeborn-client-spark-3-shaded_2.12-0.6.0-rc3.jar:?]
```
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
UT.
Closes#3401 from turboFei/fix_npe_cleaner.
Authored-by: Wang, Fei <fwang12@ebay.com>
Signed-off-by: Wang, Fei <fwang12@ebay.com>
20a629a432