`get_struct`/`set_struct!` overspecializes on field types that do nothing

[charleskawczynski]

For example,

using ClimaCore: Geometry, DataLayouts
S = ClimaCore.Geometry.Covariant3Vector{Float64}
fieldcount(S) # returns 2

Which means that for every type parameter in datalayouts that do not back data, we compile get_struct methods that do nothing:

Base.@propagate_inbounds @generated function get_struct(
    array::AbstractArray{T},
    ::Type{S},
    ::Val{D},
    start_index::CartesianIndex,
) where {T, S, D}
    tup = :(())
    for i in 1:fieldcount(S)
        push!(
            tup.args,
            :(get_struct(
                array,
                fieldtype(S, $i),
                Val($D),
                offset_index(
                    start_index,
                    Val($D),
                    $(fieldtypeoffset(T, S, Val(i))),
                ),
            )),
        )
    end
    return quote
        Base.@_propagate_inbounds_meta
        @inbounds bypass_constructor(S, $tup)
    end
end

I think that this is one of the most over-specialized methods, so we should probably fix this. I think the simple fix is to check if fieldcount(fieldtype(S, $i)) == 0 before recursing to the next method.