Fix missing NULL argument to g_strconcat.
Fix missing NULL argument to g_strconcat.

https://bugzilla.gnome.org/show_bug.cgi?id=644185

GNOME/at-spi2-core,gloob/gloob-s-at-spi2-core,gloob/gloob-s-at-spi2-core,GNOME/at-spi2-core,gloob/gloob-s-at-spi2-core,GNOME/at-spi2-core