xmlnode / XMPP mood API generate bogus XML

[darkrain42]

As reported on IRC, we sent:

<iq type='set' id='purple82a35e1a'><pubsub xmlns='http://jabber.org/protocol/pubsub'><publish node='http://jabber.org/protocol/mood'><item><mood xmlns='http://jabber.org/protocol/mood'></></mood></item></publish></pubsub></iq>

Looking through the code, this seems to be the result of passing "" to xmlnode_new_child. Both the mood API and xmlnode should be updated to prevent this from happening (after testing to confirm that's how this came about)

[darkrain42]

Reporter indicates disabling pbar plugin resolved this, so checking how it calls the mood stuff should be helpful.

[darkrain42@…]

(In [fb8d8417592c55ace530f527d2440c3240235342]):
jabber: Check for empty string when setting mood.

The xmlnode API didn't check for the empty string ("") as node name, which leads to...broken XML. I added those checks to xmlnode, and fixed this occurrence in usermood.

Fixes #14342