Fix of invalid presence stanza parsing.

I found this strange and I think incorrect behaviour in smack: if you got presence without <show> element mode will be null. But if there is available value it is more correct to fill it as available, otherwise it blows minds of library users.

Tinder for example does not have available state of Presence.Show element, and uses null value as available, and it stated in it's javadoc (http://www.igniterealtime.org/builds/tinder/docs/latest/javadoc/org/xmpp/packet/Presence.Show.html)

SA. http://xmpp.org/rfcs/rfc6121.html#presence-syntax-children-show