Export of null authors fails.

Raised from https://support.atlassian.com/browse/BSP-1090.

We need to put in a null check in authormapper class to prevent export of null authors -

2008-07-26 11:43:13,869 ERROR [TP-Processor35] [Export] java.lang.IllegalArgumentException: Adding text to an XML document must not be null
java.lang.IllegalArgumentException: Adding text to an XML document must not be null
at org.dom4j.DocumentFactory.createText(DocumentFactory.java:142)
at org.dom4j.tree.AbstractElement.addText(AbstractElement.java:1286)
at com.atlassian.bamboo.migration.AuthorMapper.exportXml(AuthorMapper.java:44)
at com.atlassian.bamboo.migration.XmlMigrator.exportXml(XmlMigrator.java:221)
at com.atlassian.bamboo.migration.XmlMigrator$$FastClassByCGLIB$$b6578fc7.invoke(<generated>)
at net.sf.cglib.proxy.MethodProxy.invoke(MethodProxy.java:149)
at org.springframework.aop.framework.Cglib2AopProxy$CglibMethodInvocation.invokeJoinpoint(Cglib2AopProxy.java:694)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)