For given node, it returns extended name of the node,
i.e. the name of given attribute or element with namespace prefix replaced
with namespace URI string.
If the argument is node-set, first node of the node-set will be considered.
Empty string is returned if the argument is an empty node-set, a node without
name or if the argument is not a node.If the argument is omitted, context node is used instead as if it is a node-set of one element.