Fix peas_engine_get_loaded_plugins() when there are not plugins.
Fix peas_engine_get_loaded_plugins() when there are not plugins.

Before would return non-NULL junk now it returns NULL.
chergert/libpeas,gregier/libpeas,gregier/libpeas,Distrotech/libpeas,gregier/libpeas,chergert/libpeas,chergert/libpeas,gregier/libpeas,GNOME/libpeas,Distrotech/libpeas,Distrotech/libpeas,GNOME/libpeas