Define copyto! for ShapedView that's no-op for 0-length arrays

[colinxs]

using Shapes, BenchmarkTools

@inline maybecopyto!(shape, dst, src) = _maybecopyto!(Length(shape), dst, src)
@inline _maybecopyto!(::Shapes.Length{0}, dst, src) = dst
@inline _maybecopyto!(::Shapes.Length, dst, src) = copyto!(dst, src)

function bench()
    dst = rand(9,0)
    src = rand(9,0)
    shape = MatrixShape(Float64, 9, 0)
    @btime maybecopyto!($shape, $dst, $src)
    @btime copyto!($dst, $src)
end

julia> bench()
  2.794 ns (0 allocations: 0 bytes)
  4.331 ns (0 allocations: 0 bytes)
end