Fix NPE on DatasetUpdateComputation

Computation: dataset-update fails last record: exp-dataset-update-00:+171797, after retries.
java.lang.NullPointerException: null
	at org.nuxeo.ai.bulk.DatasetUpdateComputation.isEndOfBatch(DatasetUpdateComputation.java:159) ~[nuxeo-ai-model-2.7.1.jar:?]
	at org.nuxeo.ai.bulk.DatasetUpdateComputation.processRecord(DatasetUpdateComputation.java:84) ~[nuxeo-ai-model-2.7.1.jar:?]
	at org.nuxeo.lib.stream.computation.log.ComputationRunner.lambda$processRecordWithRetry$10(ComputationRunner.java:417) ~[nuxeo-stream-10.10-HF65.jar:?]
	at net.jodah.failsafe.Functions$10.call(Functions.java:252) ~[failsafe-1.1.0.jar:1.1.0]
	at net.jodah.failsafe.SyncFailsafe.call(SyncFailsafe.java:145) [failsafe-1.1.0.jar:1.1.0]
	at net.jodah.failsafe.SyncFailsafe.run(SyncFailsafe.java:81) [failsafe-1.1.0.jar:1.1.0]
	at org.nuxeo.lib.stream.computation.log.ComputationRunner.processRecordWithRetry(ComputationRunner.java:417) [nuxeo-stream-10.10-HF65.jar:?]
	at org.nuxeo.lib.stream.computation.log.ComputationRunner.processRecord(ComputationRunner.java:402) [nuxeo-stream-10.10-HF65.jar:?]
	at org.nuxeo.lib.stream.computation.log.ComputationRunner.processLoop(ComputationRunner.java:291) [nuxeo-stream-10.10-HF65.jar:?]
	at org.nuxeo.lib.stream.computation.log.ComputationRunner.runOnce(ComputationRunner.java:237) [nuxeo-stream-10.10-HF65.jar:?]
	at org.nuxeo.lib.stream.computation.log.ComputationRunner.run(ComputationRunner.java:204) [nuxeo-stream-10.10-HF65.jar:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_342]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_342]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_342]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_342]
	at java.lang.Thread.run(Thread.java:750) [?:1.8.0_342]