collect_structarray does not work with empty iterator

julia> StructVector(x for x in [] if false)
ERROR: UndefVarError: T not defined
Stacktrace:
 [1] buildfromschema(::Function, ::Core.TypeofBottom) at /home/takafumi/.julia/dev/StructArrays/src/lazy.jl:25
 [2] (::StructArrays.StructArrayInitializer{StructArrays.var"#41#43",typeof(StructArrays.default_array)})(::Type, ::Tuple{Int64}) at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:13
 [3] #collect_empty_structarray#104 at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:52 [inlined]
 [4] #collect_structarray#103 at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:47 [inlined]
 [5] #collect_structarray#101 at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:41 [inlined]
 [6] #StructArray#40 at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:81 [inlined]
 [7] StructArray at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:81 [inlined]
 [8] #StructVector#8 at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:49 [inlined]
 [9] StructArray{T,1,C,I} where I where C<:Union{Tuple, NamedTuple} where T(::Base.Generator{Base.Iterators.Filter{var"#128#130",Array{Any,1}},var"#127#129"}) at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:49

I think it makes sense to at least make the error message less cryptic.

If you want to make it return something, my preference is to return an array of Union{} element type. Related discussion in: https://github.com/JuliaData/TypedTables.jl/issues/55

Reproduce the empty-iterator case with StructVector(x for x in [] if false), then inspect src/collect.jl and src/lazy.jl at the stack-trace locations. Review the related TypedTables.jl discussion to determine whether the result should use Union{} or only provide a clearer error; done means the empty case has intentional, tested behavior.