Improve handling of vectors of vectors

[bkamins]

Fixes #354.

[bkamins]

Current behavior under Julia nightly:

julia> Tables.columntable(Vector{Any}[])
(ref = MemoryRef{Any}[], size = Tuple{Int64}[])

julia> T = typeof(view(Vector{Any}[], :))
SubArray{Vector{Any}, 1, Vector{Vector{Any}}, Tuple{Base.Slice{Base.OneTo{Int64}}}, true}

julia> Tables.columntable(T[])
(parent = Vector{Vector{Any}}[], indices = Tuple{Base.Slice{Base.OneTo{Int64}}}[], offset1 = Int64[], stride1 = Int64[])

[nalimilan]

Maybe we should apply this to any AbstractVector? There's a small risk of breaking something, but it would sound more consistent.

[bkamins]

We patched this in DataFrames.jl in JuliaData/DataFrames.jl#3435, so making adjustments here is not critical for DataFrames.jl.