Fix from_chainerx to accept None
Fix from_chainerx to accept None

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