BACKPORT REPO-145 NULL field cannot be persisted when handle is checked out

When a field is set to null in an OCM workflow, the persistence may fail.

Corresponding stack trace:

[INFO] [talledLocalContainer] javax.jcr.version.VersionException: Unable to perform operation. Node is checked-in.
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.ItemValidator.checkCondition(ItemValidator.java:284)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.ItemValidator.checkModify(ItemValidator.java:248)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.ItemRemoveOperation.perform(ItemRemoveOperation.java:70)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.session.SessionState.perform(SessionState.java:200)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.ItemImpl.perform(ItemImpl.java:91)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.ItemImpl.remove(ItemImpl.java:322)
[INFO] [talledLocalContainer] at org.hippoecm.repository.decorating.ItemDecorator.remove(ItemDecorator.java:154)
[INFO] [talledLocalContainer] at org.hippoecm.repository.impl.NodeDecorator.remove(NodeDecorator.java:102)
[INFO] [talledLocalContainer] at org.hippoecm.repository.ocm.ValueMappingStrategy.storeObjectField(ValueMappingStrategy.java:751)
[INFO] [talledLocalContainer] at org.hippoecm.repository.ocm.ValueMappingStrategy.store(ValueMappingStrategy.java:672)
[INFO] [talledLocalContainer] at org.hippoecm.repository.ocm.ValueMappingStrategy.update(ValueMappingStrategy.java:584)
[INFO] [talledLocalContainer] at org.hippoecm.repository.ocm.UpdateFieldManager.storeObjectField(UpdateFieldManager.java:53)
[INFO] [talledLocalContainer] at org.datanucleus.state.AbstractStateManager.providedObjectField(AbstractStateManager.java:1043)
[INFO] [talledLocalContainer] at org.hippoecm.repository.reviewedactions.BasicReviewedActionsWorkflowImpl.jdoProvideField(BasicReviewedActionsWorkflowImpl.java)
[INFO] [talledLocalContainer] at org.hippoecm.repository.reviewedactions.FullReviewedActionsWorkflowImpl.jdoProvideField(FullReviewedActionsWorkflowImpl.java)
[INFO] [talledLocalContainer] at org.hippoecm.repository.reviewedactions.BasicReviewedActionsWorkflowImpl.jdoProvideFields(BasicReviewedActionsWorkflowImpl.java)
[INFO] [talledLocalContainer] at org.datanucleus.jdo.state.JDOStateManagerImpl.provideFields(JDOStateManagerImpl.java:2923)
[INFO] [talledLocalContainer] at org.datanucleus.state.ObjectProviderImpl.provideFields(ObjectProviderImpl.java:65)
[INFO] [talledLocalContainer] at org.hippoecm.repository.ocm.JcrPersistenceHandler.updateObject(JcrPersistenceHandler.java:136)
[INFO] [talledLocalContainer] at org.datanucleus.jdo.state.JDOStateManagerImpl.flush(JDOStateManagerImpl.java:4841)
[INFO] [talledLocalContainer] at org.datanucleus.ObjectManagerImpl.flushInternal(ObjectManagerImpl.java:3179)
[INFO] [talledLocalContainer] at org.datanucleus.ObjectManagerImpl.flush(ObjectManagerImpl.java:3119)
[INFO] [talledLocalContainer] at org.datanucleus.ObjectManagerImpl.preCommit(ObjectManagerImpl.java:3260)
[INFO] [talledLocalContainer] at org.datanucleus.ObjectManagerImpl$2.transactionPreCommit(ObjectManagerImpl.java:324)
[INFO] [talledLocalContainer] at org.datanucleus.TransactionImpl.internalPreCommit(TransactionImpl.java:394)
[INFO] [talledLocalContainer] at org.datanucleus.TransactionImpl.commit(TransactionImpl.java:279)
[INFO] [talledLocalContainer] at org.datanucleus.jdo.JDOTransaction.commit(JDOTransaction.java:90)
[INFO] [talledLocalContainer] at org.hippoecm.repository.impl.DocumentManagerImpl.putObject(DocumentManagerImpl.java:164)
[INFO] [talledLocalContainer] at org.hippoecm.repository.impl.WorkflowManagerImpl$WorkflowInvocationHandler.invoke(WorkflowManagerImpl.java:606)
[INFO] [talledLocalContainer] at $Proxy29.commitEditableInstance(Unknown Source)