Move spi_object_append_null_reference and adjust its usage
Move spi_object_append_null_reference and adjust its usage

This prevents a compilation failure on OpenSolaris

GNOME/at-spi2-core,Distrotech/at-spi2-atk,GNOME/at-spi2-core,Distrotech/at-spi2-atk,GNOME/at-spi2-core,GNOME/at-spi2-atk