map errors if function returns nothing

[bencottier]

Bit of an edge case, but seems valid.

If using map on a KeyedDataset, and the block returns nothing, it errors, because of this line trying to assign nothing as a KeyedArray.

MWE:

julia> ds = KeyedDataset(:input => KeyedArray([1, 2]; a=[1, 2]));

julia> map(ds) do A
           println(typeof(A))
       end
KeyedArray{Int64, 1, NamedDimsArray{(:a,), Int64, 1, Vector{Int64}}, Base.RefValue{Vector{Int64}}}
ERROR: MethodError: Cannot `convert` an object of type 
  Nothing to an object of type 
  KeyedArray
Closest candidates are:
  convert(::Type{T}, ::LinearAlgebra.Factorization) where T<:AbstractArray at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/factorization.jl:58
  convert(::Type{T}, ::T) where T<:AbstractArray at abstractarray.jl:14
  convert(::Type{T}, ::T) where T at essentials.jl:205
Stacktrace:
 [1] setindex!(dd::OrderedCollections.LittleDict{Tuple, KeyedArray, Vector{Tuple}, Vector{KeyedArray}}, value::Nothing, key::Tuple{Symbol})
   @ OrderedCollections ~/.julia/packages/OrderedCollections/cP9uu/src/little_dict.jl:219
 [2] map!(f::var"#9#10", dest::KeyedDataset, src::KeyedDataset)
   @ AxisSets ~/.julia/packages/AxisSets/IddYk/src/functions.jl:49
 [3] map(::Function, ::KeyedDataset)
   @ AxisSets ~/.julia/packages/AxisSets/IddYk/src/functions.jl:41
 [4] top-level scope
   @ REPL[16]:1

If the point of map(::KeyedDataset) is only to modify the dataset, that may be fair, but I think it should either have a fallback, or a clearer error.

[bencottier]

More generally, there are convenient uses of map that don't return KeyedArray.

E.g. checking if any components are empty: any(map(isempty, ds)) rather than any(isempty.(values(ds.data))).

[rofinn]

Yeah, I'll need to look more closely with how map figures out the return types for allocating containers.