Fix bug where Array.compute() fails on empty arrays.
Fix bug where Array.compute() fails on empty arrays.

This came up fixing the indexing bug where the slice.stop == 0.

ssanderson/dask,dask/dask,hainm/dask,pombredanne/dask,esc/dask,PhE/dask,wiso/dask,simudream/dask,blaze/dask,vikhyat/dask,ContinuumIO/dask,mikegraham/dask,jayhetee/dask,gameduell/dask,jakirkham/dask,jakirkham/dask,marianotepper/dask,marianotepper/dask,minrk/dask,clarkfitzg/dask,hainm/dask,jcrist/dask,pombredanne/dask,freeman-lab/dask,blaze/dask,cowlicks/dask,mraspaud/dask,jayhetee/dask,dask/dask,wiso/dask,vikhyat/dask,esc/dask,clarkfitzg/dask,mrocklin/dask,PhE/dask,mraspaud/dask,ContinuumIO/dask,jcrist/dask,chrisbarber/dask,freeman-lab/dask,simudream/dask,ssanderson/dask,cpcloud/dask,minrk/dask,mrocklin/dask