Do nothing if empty list of arrays is given in Backward()
Do nothing if empty list of arrays is given in Backward()

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