Use None as null pointer
Use None as null pointer

hvy/chainer,sou81821/chainer,benob/chainer,t-abe/chainer,okuta/chainer,jnishi/chainer,ktnyt/chainer,1986ks/chainer,tigerneil/chainer,tkerola/chainer,jnishi/chainer,rezoo/chainer,chainer/chainer,ktnyt/chainer,kikusu/chainer,AlpacaDB/chainer,truongdq/chainer,anaruse/chainer,okuta/chainer,AlpacaDB/chainer,hvy/chainer,chainer/chainer,pfnet/chainer,jnishi/chainer,aonotas/chainer,niboshi/chainer,niboshi/chainer,minhpqn/chainer,cupy/cupy,muupan/chainer,truongdq/chainer,wkentaro/chainer,keisuke-umezawa/chainer,tscohen/chainer,keisuke-umezawa/chainer,ytoyama/yans_chainer_hackathon,wkentaro/chainer,cupy/cupy,wkentaro/chainer,ktnyt/chainer,hvy/chainer,kashif/chainer,Kaisuke5/chainer,cupy/cupy,chainer/chainer,muupan/chainer,chainer/chainer,cupy/cupy,cemoody/chainer,ronekko/chainer,t-abe/chainer,benob/chainer,ysekky/chainer,sinhrks/chainer,wkentaro/chainer,okuta/chainer,hvy/chainer,laysakura/chainer,niboshi/chainer,keisuke-umezawa/chainer,jnishi/chainer,ktnyt/chainer,kikusu/chainer,okuta/chainer,sinhrks/chainer,keisuke-umezawa/chainer,niboshi/chainer,delta2323/chainer,kiyukuta/chainer