[MUC] Fix handling of empty room topic lm_message_node_get_child_value() didn't make any difference between a missing node and an empty one. With this patch it will return "" if the node exists but has no value.

[Desktop Entry]
Encoding=UTF-8
Type=Application
Name=MCabber
GenericName=Jabber IM Client
Comment=Console Jabber Client
Exec=mcabber
Terminal=true
Categories=ConsoleOnly;System;Network;InstantMessaging;Chat;