ERROR: MethodError: no method matching zero(::Type{Union{Nothing, String}}) when trying to serialize Union{Nothing,String} fields

[jrevels]

MWE:

julia> using Arrow

julia> t = (; x = Union{Missing,NamedTuple{(:k,),Tuple{Union{Nothing,String}}}}[missing, (; k = "k")])
(x = Union{Missing, NamedTuple{(:k,), Tuple{Union{Nothing, String}}}}[missing, NamedTuple{(:k,), Tuple{Union{Nothing, String}}}(("k",))],)

julia> a = Arrow.Table(Arrow.tobuffer(t))
ERROR: MethodError: no method matching zero(::Type{Union{Nothing, String}})
Closest candidates are:
  zero(::Union{Type{P}, P}) where P<:Dates.Period at /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/Dates/src/periods.jl:53
  zero(::AbstractIrrational) at irrationals.jl:150
  zero(::CartesianIndex{N}) where N at multidimensional.jl:106
  ...
Stacktrace:
  [1] default(T::Type)
    @ ArrowTypes ~/.julia/packages/ArrowTypes/dkiHE/src/ArrowTypes.jl:305
  [2] getindex
    @ ~/.julia/packages/Arrow/QsQ3U/src/arraytypes/struct.jl:80 [inlined]
  [3] iterate
    @ ./abstractarray.jl:1167 [inlined]
  [4] iterate
    @ ./abstractarray.jl:1165 [inlined]
  [5] iterate
    @ ./iterators.jl:167 [inlined]
  [6] iterate
    @ ./iterators.jl:166 [inlined]
  [7] todense(#unused#::Type{Arrow.UnionT{Arrow.Flatbuf.UnionModes.Dense, nothing, Tuple{Nothing, String}}}, x::Arrow.ToStruct{Union{Nothing, String}, 1, Vector{Union{Missing, NamedTuple{(:k,), Tuple{Union{Nothing, String}}}}}})
    @ Arrow ~/.julia/packages/Arrow/QsQ3U/src/arraytypes/unions.jl:112
  [8] arrowvector(::ArrowTypes.UnionKind, x::Arrow.DenseUnionVector{Arrow.ToStruct{Union{Nothing, String}, 1, Vector{Union{Missing, NamedTuple{(:k,), Tuple{Union{Nothing, String}}}}}}, Tuple{Nothing, String}}, i::Int64, nl::Int64, fi::Int64, de::Dict{Int64, Any}, ded::Vector{Arrow.DictEncoding}, meta::Nothing; kw::Base.Pairs{Symbol, Union{Nothing, Integer}, NTuple{6, Symbol}, NamedTuple{(:dictencode, :maxdepth, :denseunions, :compression, :largelists, :dictencodenested), Tuple{Bool, Int64, Bool, Nothing, Bool, Bool}}})
    @ Arrow ~/.julia/packages/Arrow/QsQ3U/src/arraytypes/unions.jl:219

This might be tangential, but it's a bit confusing to me when a method for default is actually needed or not; it has a docstring, but it doesn't show up in the manual IIRC

[quinnj]

This is fixed