Bug in copyto!

[bkamins]

The following code currently fails on master:

julia> copyto!([1,2], view(Int[],Int[]))
ERROR: BoundsError: attempt to access 2-element Array{Int64,1} at index [0]
Stacktrace:
 [1] throw_boundserror(::Array{Int64,1}, ::Tuple{Int64}) at ./abstractarray.jl:537
 [2] checkbounds at ./abstractarray.jl:502 [inlined]
 [3] copyto!(::Array{Int64,1}, ::SubArray{Int64,1,Array{Int64,1},Tuple{Array{Int64,1}},false}) at ./multidimensional.jl:975
 [4] top-level scope at REPL[23]:1

The reason is here: https://github.com/JuliaLang/julia/blob/master/base/multidimensional.jl#L975
and is most likely caused by #34049.

The problem is that empty source is incorrectly handled in this part of code.

CC @pablosanjose @timholy