NPE fix in core engine (TaskHandler)

See https://developer.jboss.org/thread/239791 for more information.

In short, this line in the org.jbpm.bpmn2.xml.TaskHandler class can cause an NPE because 'subNode' may be null.

Furthermore, the comment suggests that a null check be done on subNode.getNextSibling(). This makes sense, given that the next sibling is retrieved 4 lines under the referenced line.