Avoid piracy in cat

[jishnub]

Currently, on master

julia> cat(dims=1)
Any[]

julia> using StructArrays

julia> cat(dims=1)
ERROR: MethodError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
Stacktrace:
  [1] reduce_empty(op::Base.MappingRF{typeof(eltype), Base.BottomRF{typeof(promote_type)}}, #unused#::Type{Union{}})
    @ Base ./reduce.jl:356
  [2] reduce_empty_iter(op::Base.MappingRF{typeof(eltype), Base.BottomRF{typeof(promote_type)}}, itr::Tuple{}, #unused#::Base.HasEltype)
    @ Base ./reduce.jl:379
  [3] reduce_empty_iter(op::Base.MappingRF{typeof(eltype), Base.BottomRF{typeof(promote_type)}}, itr::Tuple{})
    @ Base ./reduce.jl:378
  [4] foldl_impl(op::Base.MappingRF{typeof(eltype), Base.BottomRF{typeof(promote_type)}}, nt::Base._InitialValue, itr::Tuple{})
    @ Base ./reduce.jl:49
  [5] mapfoldl_impl(f::typeof(eltype), op::typeof(promote_type), nt::Base._InitialValue, itr::Tuple{})
    @ Base ./reduce.jl:44
  [6] mapfoldl(f::Function, op::Function, itr::Tuple{}; init::Base._InitialValue)
    @ Base ./reduce.jl:170
  [7] mapfoldl
    @ ./reduce.jl:170 [inlined]
  [8] #mapreduce#263
    @ ./reduce.jl:302 [inlined]
  [9] mapreduce(f::Function, op::Function, itr::Tuple{})
    @ Base ./reduce.jl:302
 [10] cat(; kwargs::Base.Pairs{Symbol, Int64, Tuple{Symbol}, NamedTuple{(:dims,), Tuple{Int64}}})
    @ StructArrays ~/Dropbox/JuliaPackages/StructArrays.jl/src/structarray.jl:450
 [11] top-level scope
    @ REPL[2]:1

This PR fixes this by avoiding the accidental type piracy

[piever]

Well spotted!