Issue

We use XmlDataExporter for saving journal, page and large-message data from a stopped instance of HornetQ in an XML format.

If a messages has a String header property with a null value, for example:

 message.setStringProperty("boot_size", null)

we see an exception like the following:

java.lang.NullPointerException
        at org.hornetq.core.persistence.impl.journal.XmlDataExporter.printMessageProperties(XmlDataExporter.java:635)
        at org.hornetq.core.persistence.impl.journal.XmlDataExporter.printSingleMessageAsXML(XmlDataExporter.java:534)
        at org.hornetq.core.persistence.impl.journal.XmlDataExporter.printAllMessagesAsXML(XmlDataExporter.java:432)
        at org.hornetq.core.persistence.impl.journal.XmlDataExporter.printDataAsXML(XmlDataExporter.java:391)
        at org.hornetq.core.persistence.impl.journal.XmlDataExporter.writeXMLData(XmlDataExporter.java:186)
        at org.hornetq.core.persistence.impl.journal.XmlDataExporter.main(XmlDataExporter.java:173)