[BREAKING] Return 0 missing count for columns of nonmissing type

[nilshg]

This came up on Slack - as the eltype is printed in the returned table, there's little additional information in returning nothing, as the user can see from eltype whether the columns allows missing or not. Returning 0 here makes sorting the table on this column easier.

[bkamins]

Looks good apart for the new file that should not land here. In the past the distinction was required, but recently we started to show full eltype (rather than stripped one as we did in the past).

[bkamins]

As a general note:

1. I would like to highlight that we now avoid making breaking changes. Also for future readers - please make sure to add [BREAKING] in PR name if it is breaking
2. In this case I think it is ok to be breaking, as it is mildly breaking and should not actually break any production workflow (unless someone relied on the old behaviour, but because of this change I will not backport it)
3. Still I will wait for @nalimilan to approve before merging, as it is breaking

Thank you for the PR.

[bkamins]

Thank you!