JSON3.write expects the output of pairs to have length defined

[Adam B]

The pairs function from Base is only required to return an iterator, but if you define e.g. JSON3.StructType(::Type{MyType}) = JSON3.ObjectType() and Base.pairs(x::MyType) = Iterators.filter(isnothing∘last, ("foo" => "bar", "foobar" => nothing)), JSON3.write throws a MethodError:

MethodError: no method matching length(::Base.Iterators.Filter{Base.var"#56#57"{Base.var"#58#59"{typeof(isnothing)},typeof(last)},Base.Generator{NTuple{6,Symbol},MatrixProtocolClient.var"#11#12"{MatrixProtocolClient.LoginBody{MatrixProtocolClient.UserIdentifier}}}})
Closest candidates are:
  length(!Matched::Core.SimpleVector) at essentials.jl:593
  length(!Matched::Base.MethodList) at reflection.jl:849
  length(!Matched::Core.MethodTable) at reflection.jl:923
  ...

Stacktrace:
 [1] #write#68(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(JSON3.write), ::JSON3.ObjectType, ::Array{UInt8,1}, ::Int64, ::Int64, ::MatrixProtocolClient.LoginBody{MatrixProtocolClient.UserIdentifier}) at /home/adam/.julia/packages/JSON3/ItGdr/src/write.jl:129
 [2] write(::JSON3.ObjectType, ::Array{UInt8,1}, ::Int64, ::Int64, ::MatrixProtocolClient.LoginBody{MatrixProtocolClient.UserIdentifier}) at /home/adam/.julia/packages/JSON3/ItGdr/src/write.jl:127
 [3] #write#55(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(JSON3.write), ::MatrixProtocolClient.LoginBody{MatrixProtocolClient.UserIdentifier}) at /home/adam/.julia/packages/JSON3/ItGdr/src/write.jl:24
 [4] write(::MatrixProtocolClient.LoginBody{MatrixProtocolClient.UserIdentifier}) at /home/adam/.julia/packages/JSON3/ItGdr/src/write.jl:22
 [5] top-level scope at In[26]:1

JSON3 v0.1.13