implement - nil datatype - polar null type column

[lkarthee]

For addressing issues relating to null datatype.

Its not ready for merging - need code review and some help.

[lkarthee]

@josevalim @philss There are a lot of test failures - i will fix them after review ..

[josevalim]

Thank you!

Let's add some tests that to_list works and returns a list of nils. I assume to_binary and to_iovec both fail? Do they fail with a decent error message at least?

[lkarthee]

Sure I will add test cases for to_list.

Error message for to_iovec and to_binary.

** (ArgumentError) cannot convert series of dtype :null into iovec
    (explorer 0.8.0-dev) lib/explorer/series.ex:769: Explorer.Series.to_iovec/1
    #cell:2wexfollu25fyu43:14: (file)

For [1, 2, nil] to_iovec fails with :

** (RuntimeError) Generic Error: cannot invoke to_iovec on series with nils

How to fix iovec error any pointers ?

[josevalim]

Those are all expected and good. So we only need tests for to_list and we are good to go. :)

[josevalim]

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