There are some registred stock ids like gtk-discards that have no icons,
and you could also pass a non-registred stock id. Both of these means
gtk_style_context_lookup_icon_set returns NULL, which causes
a critical in gtk_icon_set_render_icon_pixbuf.
We avoid this by just making these render as EMPTY.
Treat the empty string as if it was a NULL icon name instead of
rendering GTK_STOCK_MISSING_IMAGE.
This used to be the previous behavior of GtkCellRendererPixbuf and
applications might still rely on it.
GtkIconHelper is a helper object to easily obtain a pixbuf from
different icon sources (e.g. a GIcon, an icon name, a stock id, ...).
Code is ported from GtkImage, which will be adapted in the next commit.
