NPE occurs in the method `ManagedLedgerImpl.isOffloadedNeedsDelete`.

[gaoran10]

Describe the bug

NPE occurs in the method ManagedLedgerImpl.isOffloadedNeedsDelete .

To Reproduce
Steps to reproduce the behavior:

1. Use the offloader feature.

Error Log

[bookkeeper-ml-workers-OrderedExecutor-0-0] ERROR org.apache.bookkeeper.common.util.SafeRunnable - Unexpected throwable caught 
java.lang.NullPointerException: null
	at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.isOffloadedNeedsDelete(ManagedLedgerImpl.java:1968) ~[org.apache.pulsar-managed-ledger-2.6.0-sn-candidate-2.jar:2.6.0-sn-candidate-2]
	at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.internalTrimConsumedLedgers(ManagedLedgerImpl.java:2050) ~[org.apache.pulsar-managed-ledger-2.6.0-sn-candidate-2.jar:2.6.0-sn-candidate-2]
	at org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.lambda$trimConsumedLedgersInBackground$20(ManagedLedgerImpl.java:1875) ~[org.apache.pulsar-managed-ledger-2.6.0-sn-candidate-2.jar:2.6.0-sn-candidate-2]
	at org.apache.bookkeeper.mledger.util.SafeRun$1.safeRun(SafeRun.java:32) ~[org.apache.pulsar-managed-ledger-2.6.0-sn-candidate-2.jar:2.6.0-sn-candidate-2]
	at org.apache.bookkeeper.common.util.SafeRunnable.run(SafeRunnable.java:36) [org.apache.bookkeeper-bookkeeper-common-4.10.0.jar:4.10.0]
	at org.apache.bookkeeper.common.util.OrderedExecutor$TimedRunnable.run(OrderedExecutor.java:203) [org.apache.bookkeeper-bookkeeper-common-4.10.0.jar:4.10.0]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_252]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_252]
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [io.netty-netty-common-4.1.48.Final.jar:4.1.48.Final]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_252]

Expected behavior

We should make NPE check and avoid the NPE appears.

Desktop (please complete the following information):

• OS: [e.g. Macos 10.15.5]