Added parser method used to obtain a 'null' object reference.

git-svn-id: http://yate.null.ro/svn/yate/trunk@5830 acf43c95-373e-0410-b603-e72c3f656dc1
Return null object from XML constructor if parse fails.

git-svn-id: http://yate.null.ro/svn/yate/trunk@5831 acf43c95-373e-0410-b603-e72c3f656dc1