* rsvg.c: (rsvg_entity_decl): Change so it can handle a NULL
	for the content passed in. Happens at least if the .svg file
	is empty and presumably in other cases.
	(rsvg_handle_free): Handle case where pixbuf is NULL.
	* test-rsvg.c: (main): Don't write anything out if a NULL
	pixbuf is returned. This can happen if the .svg file is
	empty.