Avoid NPE in transformations

Using email reader the following error encountered
15:33:20,842 ERROR [BinaryConverter] Error during XL2Text conversion
org.nuxeo.ecm.core.convert.api.ConversionException: Error during XL2Text conversion
at org.nuxeo.ecm.core.convert.plugins.text.extractors.XL2TextConverter.convert(XL2TextConverter.java:71)
at org.nuxeo.ecm.core.convert.service.ConversionServiceImpl.convert(ConversionServiceImpl.java:170)
at org.nuxeo.ecm.core.convert.plugins.text.extractors.FullTextConverter.convert(FullTextConverter.java:72)
at org.nuxeo.ecm.core.convert.service.ConversionServiceImpl.convert(ConversionServiceImpl.java:170)
at org.nuxeo.ecm.core.storage.sql.BinaryConverter.getString(BinaryConverter.java:72)
at org.nuxeo.ecm.core.storage.sql.PersistenceContext.updateFulltext(PersistenceContext.java:249)
at org.nuxeo.ecm.core.storage.sql.SessionImpl.flush(SessionImpl.java:178)
at org.nuxeo.ecm.core.storage.sql.TransactionalSession.end(TransactionalSession.java:69)
at org.nuxeo.ecm.core.storage.sql.ra.ConnectionAwareXAResource.end(ConnectionAwareXAResource.java:55)
at org.jboss.resource.connectionmanager.xa.JcaXAResourceWrapper.end(JcaXAResourceWrapper.java:58)
at com.arjuna.ats.internal.jta.resources.arjunacore.XAResourceRecord.topLevelOnePhaseCommit(XAResourceRecord.java:633)
at com.arjuna.ats.arjuna.coordinator.BasicAction.onePhaseCommit(BasicAction.java:2619)
at com.arjuna.ats.arjuna.coordinator.BasicAction.End(BasicAction.java:1779)
at com.arjuna.ats.arjuna.coordinator.TwoPhaseCoordinator.end(TwoPhaseCoordinator.java:88)
at com.arjuna.ats.arjuna.AtomicAction.commit(AtomicAction.java:177)
at com.arjuna.ats.internal.jta.transaction.arjunacore.TransactionImple.commitAndDisassociate(TransactionImple.java:1389)
at com.arjuna.ats.internal.jta.transaction.arjunacore.BaseTransaction.commit(BaseTransaction.java:135)
at com.arjuna.ats.jbossatx.BaseTransactionManagerDelegate.commit(BaseTransactionManagerDelegate.java:87)
at org.jboss.aspects.tx.TxPolicy.endTransaction(TxPolicy.java:175)
at org.jboss.aspects.tx.TxPolicy.invokeInOurTx(TxPolicy.java:87)
at org.jboss.aspects.tx.TxInterceptor$Required.invoke(TxInterceptor.java:191)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.aspects.tx.TxPropagationInterceptor.invoke(TxPropagationInterceptor.java:95)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.ejb3.stateful.StatefulInstanceInterceptor.invoke(StatefulInstanceInterceptor.java:83)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.aspects.security.RoleBasedAuthorizationInterceptor.invoke(RoleBasedAuthorizationInterceptor.java:166)
at org.jboss.ejb3.security.RoleBasedAuthorizationInterceptor.invoke(RoleBasedAuthorizationInterceptor.java:115)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.aspects.security.AuthenticationInterceptor.invoke(AuthenticationInterceptor.java:77)
at org.jboss.ejb3.security.Ejb3AuthenticationInterceptor.invoke(Ejb3AuthenticationInterceptor.java:110)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.ejb3.ENCPropagationInterceptor.invoke(ENCPropagationInterceptor.java:46)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.ejb3.asynchronous.AsynchronousInterceptor.invoke(AsynchronousInterceptor.java:106)
at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:101)
at org.jboss.ejb3.stateful.StatefulContainer.localInvoke(StatefulContainer.java:206)
at org.jboss.ejb3.stateful.StatefulLocalProxy.invoke(StatefulLocalProxy.java:119)
at $Proxy270.createDocument(Unknown Source)
at com.nuxeo.projects.<proj>.mail.action.CreateDocumentsAction.execute(CreateDocumentsAction.java:57)
at org.nuxeo.ecm.platform.mail.action.Visitor.visit(Visitor.java:86)
at com.nuxeo.projects.<proj>.mail.listener.ProjMailListener.handleEvent(GiecbMailListener.java:160)
at org.nuxeo.ecm.core.listener.impl.DefaultEventService.fireEvent(DefaultEventService.java:131)
at org.nuxeo.ecm.core.listener.impl.DefaultEventService.notifyEventListeners(DefaultEventService.java:116)
at org.nuxeo.ecm.core.event.compat.CompatibilityListener.handleEvent(CompatibilityListener.java:52)
at org.nuxeo.ecm.core.event.impl.EventServiceImpl.fireEvent(EventServiceImpl.java:136)
at org.nuxeo.ecm.platform.scheduler.core.EventJob.execute(EventJob.java:82)
at org.quartz.core.JobRunShell.run(JobRunShell.java:203)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:520)
Caused by: org.nuxeo.ecm.core.api.WrappedException: Exception: java.lang.NullPointerException. message: null
at org.nuxeo.ecm.core.convert.plugins.text.extractors.XL2TextConverter.appendTextFromCell(XL2TextConverter.java:94)
at org.nuxeo.ecm.core.convert.plugins.text.extractors.XL2TextConverter.convert(XL2TextConverter.java:64)
... 48 more