Support None in dask.ghost.boundaries.
Support None in dask.ghost.boundaries.

mraspaud/dask,jcrist/dask,pombredanne/dask,jakirkham/dask,vikhyat/dask,PhE/dask,blaze/dask,clarkfitzg/dask,PhE/dask,ContinuumIO/dask,pombredanne/dask,mikegraham/dask,dask/dask,chrisbarber/dask,gameduell/dask,mraspaud/dask,dask/dask,cpcloud/dask,cowlicks/dask,clarkfitzg/dask,mrocklin/dask,jakirkham/dask,mrocklin/dask,jcrist/dask,ContinuumIO/dask,vikhyat/dask,blaze/dask