gconfig: fix null pointer warning

In gconfig if you enable "Show all options", you'll see some "(null)"
config options, and clicking those options triggers a warning:

(gconf:9368): Gtk-CRITICAL **: gtk_text_buffer_insert_with_tags: assertion `text != NULL' failed

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>