Unserializable histogram generated if computed without fill call

[yimuchen]

Originally posted here: https://github.com/scikit-hep/hist/issues/586 And I found that this issue can be replicated entirely upstream using just dask-histogram

import pickle

import boost_histogram
import dask
import dask_histogram.boost
import numpy

h = dask_histogram.boost.Histogram(boost_histogram.axis.Regular(10, 0, 1))
# h.fill( # Toggle to include a fill call
#     dask.array.from_array(numpy.zeros(shape=(10))),
#     weight=dask.array.from_array(numpy.zeros(shape=(10))),
# )

o = dask.compute(h)
pickle.dump(o, open("hist_dask_test.pkl", "wb"))

Without a fill call, the resulting histogram will have dangling a self._dask field that breaks serialization.