Unpacking optional components throws

[ameligrana]

MWE:

julia> using Ark

julia> struct A 
           x::Float64
       end

julia> struct B end

julia> world = World(A, B)
World(entities=0, comp_types=(A, B))

julia> new_entity!(world, (A(1.),))
Entity(2, 0)

julia> q = Query(world, (A,); optional=(B,))
Query((A); optional=(B))

julia> for t in q
           @unpack (entities, a, b) = t
       end
ERROR: MethodError: no method matching unpack(::Nothing)

Closest candidates are:
  unpack(::SubArray)
   @ Ark ~/.julia/dev/Ark/src/fieldsview.jl:24
  unpack(::FieldViews.FieldViewable{T}) where T
   @ Ark ~/.julia/dev/Ark/src/fieldsview.jl:18
  unpack(::Ark._StructArrayView)
   @ Ark ~/.julia/dev/Ark/src/structarray_interface.jl:152

Stacktrace:
 [1] top-level scope
   @ ./REPL[14]:2

[mlange-42]

Hm, at first glance I thought we could just implement unpack for Nothing as a fall-through, but this does not work as we still need to consider the number of struct fields.