Avoid returning empty dict (#442)
Avoid returning empty dict (#442)

For some reason this causes a performance spike in Tornado
amosonn/distributed,mrocklin/distributed,broxtronix/distributed,dask/distributed,dask/distributed,broxtronix/distributed,amosonn/distributed,dask/distributed,mrocklin/distributed,amosonn/distributed,mrocklin/distributed,broxtronix/distributed,blaze/distributed,dask/distributed,blaze/distributed