Update nulls in categorify encode to array instead of index value

[oliverholworthy]

Follow-up to #1692

Update nulls in categorify _encode function to array instead of index type.

This fixes an error we're seeing in our container builds with newer versions of cupy.

import cupy
import cudf

labels = cupy.array([3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 5, 5, 4, 4, 4])
nulls = cudf.Index([5, 6, 7, 8, 9], dtype='int64')
labels[nulls] = 1
# =>
# TypeError: no implementation found for 'numpy.min_scalar_type' on types that implement __array_function__: [<class 'cudf.core.index.Int64Index'>]

[github-actions]

Documentation preview

https://nvidia-merlin.github.io/NVTabular/review/pr-1836