Handle creation of empty ComponentArrays

[bgroenks96]

In some cases (such as with parameter handling) it can be useful to allow for empty ComponentArrays. This currently fails:

ComponentArray()

produces the error:

ERROR: ArgumentError: reducing over an empty collection is not allowed
Stacktrace:
  [1] _empty_reduce_error()
    @ Base ./reduce.jl:299
  [2] reduce_empty(#unused#::typeof(Base.add_sum), #unused#::Core.TypeofBottom)
    @ Base ./reduce.jl:319
  [3] reduce_empty(op::Base.BottomRF{typeof(Base.add_sum)}, #unused#::Type{Union{}})
    @ Base ./reduce.jl:328
  [4] reduce_empty_iter
    @ ./reduce.jl:355 [inlined]
  [5] reduce_empty_iter
    @ ./reduce.jl:354 [inlined]
  [6] foldl_impl(op::Base.BottomRF{typeof(Base.add_sum)}, nt::Base._InitialValue, itr::Tuple{})
    @ Base ./reduce.jl:49
  [7] mapfoldl_impl(f::typeof(identity), op::typeof(Base.add_sum), nt::Base._InitialValue, itr::Tuple{})
    @ Base ./reduce.jl:44
  [8] mapfoldl(f::Function, op::Function, itr::Tuple{}; init::Base._InitialValue)
    @ Base ./reduce.jl:160
  [9] mapfoldl
    @ ./reduce.jl:160 [inlined]
 [10] #mapreduce#218
    @ ./reduce.jl:287 [inlined]
 [11] mapreduce
    @ ./reduce.jl:287 [inlined]
 [12] #sum#221
    @ ./reduce.jl:501 [inlined]
 [13] sum(f::Function, a::Tuple{})
    @ Base ./reduce.jl:501
 [14] sum(a::Tuple{}; kw::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Base ./reduce.jl:528
 [15] sum(a::Tuple{})
    @ Base ./reduce.jl:528
 [16] |>(x::Tuple{}, f::typeof(sum))
    @ Base ./operators.jl:858
 [17] recursive_length(nt::NamedTuple{(), Tuple{}})
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/utils.jl:78
 [18] make_idx(data::Vector{Any}, nt::NamedTuple{(), Tuple{}}, last_val::Int64)
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/componentarray.jl:135
 [19] make_carray_args(A::Type{Vector{T} where T}, nt::NamedTuple{(), Tuple{}})
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/componentarray.jl:129
 [20] make_carray_args(nt::NamedTuple{(), Tuple{}})
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/componentarray.jl:123
 [21] ComponentArray(nt::NamedTuple{(), Tuple{}})
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/componentarray.jl:62
 [22] ComponentArray(; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/componentarray.jl:65
 [23] ComponentArray()
    @ ComponentArrays ~/.julia/packages/ComponentArrays/6k0iO/src/componentarray.jl:65
 [24] top-level scope
    @ REPL[3]:1

[jonniedie]

I think this should be easy to add. I'm curious to see a situation where this would be useful, though. Since ComponentArrays are parameterized by their data indices, you can't push! to them or anything like.

[bgroenks96]

I use ComponentArray for parameters as well as state variables. I may have just overtyped things, but this means I need an empty ComponentArray to handle the case where there are no parameters.

[bgroenks96]

I currently use a nasty hack by manually constructing the axis.