Fix warning when no favicon is available
(epiphany:60012): Gdk-CRITICAL **: 16:26:36.854: gdk_cairo_surface_create_from_pixbuf: assertion 'GDK_IS_PIXBUF (pixbuf)' failed
(epiphany:60012): Gdk-CRITICAL **: 16:26:36.854: gdk_cairo_surface_create_from_pixbuf: assertion 'GDK_IS_PIXBUF (pixbuf)' failed