Fix Union{} corner case in isiterabletable

[bkamins]

This is a corner case as Union{} <: NamedTuple but I guess you do not want to support Union{} eltype to be true in this test?

[bkamins]

@davidanthoff - is this OK for you?

[bkamins]

@davidanthoff - is is OK to be merged?

[davidanthoff]

I think I don't fully understand how we could end up in this situation. Does it make sense for something to return HasEltype() and then return Union{} as the element type? There are no values that are of type Union{}, right? So wouldn't it make more sense for a source to just say it doesn't have an element type in that case?

[bkamins]

See the linked issue in Tables.jl for details. The offending test is for Matrix{Union{}}(undef, 2, 3) and it is allowed in Base (not useful, but valid, and it is tested against in Tables.jl).

[bkamins]

It would be great if it could be merged (or rejected) and if it is merged - a new release of TableTraits.jl was tagged.
The reason is that we need a change in DataFrames.jl which requires a change (and release) in Tables.jl, which in turn requires a decision in this PR.

Thank you!

[bkamins]

Is there any decision on this issue?

[davidanthoff]

Sorry that this took so long, I had to just give up on certain topics in the fall, too many things going on at work :) I'm merging this as soon as tests pass and will then tag a patch release.

[bkamins]

Thank you!