Fix array python binding for nullptr to graph id conversion
Fix array python binding for nullptr to graph id conversion

jnishi/chainer,jnishi/chainer,hvy/chainer,wkentaro/chainer,ktnyt/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer,keisuke-umezawa/chainer,ktnyt/chainer,chainer/chainer,wkentaro/chainer,pfnet/chainer,okuta/chainer,hvy/chainer,chainer/chainer,hvy/chainer,ktnyt/chainer,niboshi/chainer,keisuke-umezawa/chainer,tkerola/chainer,ktnyt/chainer,niboshi/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer,jnishi/chainer,keisuke-umezawa/chainer,jnishi/chainer,wkentaro/chainer,okuta/chainer,okuta/chainer,niboshi/chainer