Fix NPE while reordering read-sequence for local-bookie ensemble policy

Fix NPE while reordering read-sequence for local-bookie ensemble policy

### Motivation

When Bookie sanity and autoreovery use the same conf file which has flag `reorderReadSequenceEnabled=true` then bookie-sanity command throws NPE as `LocalBookieEnsemblePlacementPolicy::reorderReadLACSequence` returns null writesets which causes the sanity failure.

```
00:46:46.202 [BookKeeperClientWorker-OrderedExecutor-11-0] ERROR o.a.b.common.util.SafeRunnable       - Unexpected throwable caught
java.lang.NullPointerException: null
	at org.apache.bookkeeper.client.PendingReadOp$SequenceReadRequest.sendNextRead(PendingReadOp.java:399)
	at org.apache.bookkeeper.client.PendingReadOp$SequenceReadRequest.read(PendingReadOp.java:385)
	at org.apache.bookkeeper.client.PendingReadOp.initiate(PendingReadOp.java:529)
	at org.apache.bookkeeper.client.LedgerRecoveryOp.doRecoveryRead(LedgerRecoveryOp.java:148)
	at org.apache.bookkeeper.client.LedgerRecoveryOp.access$000(LedgerRecoveryOp.java:37)
	at org.apache.bookkeeper.client.LedgerRecoveryOp$1.readLastConfirmedDataComplete(LedgerRecoveryOp.java:109)
	at org.apache.bookkeeper.client.ReadLastConfirmedOp.readEntryComplete(ReadLastConfirmedOp.java:135)
	at org.apache.bookkeeper.proto.PerChannelBookieClient$ReadCompletion$1.readEntryComplete(PerChannelBookieClient.java:1829)
	at org.apache.bookkeeper.proto.PerChannelBookieClient$ReadCompletion.handleReadResponse(PerChannelBookieClient.java:1910)
	at org.apache.bookkeeper.proto.PerChannelBookieClient$ReadCompletion.handleV3Response(PerChannelBookieClient.java:1885)
	at org.apache.bookkeeper.proto.PerChannelBookieClient$3.safeRun(PerChannelBookieClient.java:1446)
	at org.apache.bookkeeper.common.util.SafeRunnable.run(SafeRunnable.java:36)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)

```

### Modification
Fix NPE for local ensemble policy while reading entry with `reorderReadSequenceEnabled` flag enabled.

Reviewers: Andrey Yegorov <None>, Enrico Olivelli <eolivelli@gmail.com>, Nicolò Boschi <boschi1997@gmail.com>

This closes #3127 from rdhabalia/repl_seq