Allow stack on an empty collection of SArrays

[jariji]

julia> stack(SVector{3,Int}[]; dims=1)
ERROR: ArgumentError: `stack` on an empty collection is not allowed

This could work and it would be convenient for handling the empty case. Thoughts?

[mcabbott]

Xref JuliaLang/julia#55735 about why this was removed from the original stack PR.

At an earlier point, it did infer things like stack(empty!([(1,2,3)])) to be 3×0 Matrix. The same could have worked for this Vector{SVector} example (not sure if it did). But these paths gave type-instabilities... where normally stack(xs) calls similar(xs[1], ...), the empty case can't do that & wanted methods of similar(eltype(xs), ...) which don't exist.

In the stacktrace from the above error, the last function is _empty_stack which used to implement this, and now just always throws. It might be possible for StaticArrays to overload this, to catch only types it owns & hence not produce instabilities on Base / StructArrays.