var of empty vector of arrays should be vector of NaNs

[yha]

The var function operates pointwise on array arguments ( JuliaLang/julia#23897. It doesn't seem to be documented). But it fails for empty vector of arrays:

julia> using Statistics, StaticArrays

julia> var(SVector{2}.(1:5,2:6))
2-element SArray{Tuple{2},Float64,1,2} with indices SOneTo(2):
 2.5
 2.5

julia> var(SVector{2,Float64}[])
ERROR: MethodError: no method matching abs2(::SArray{Tuple{2},Float64,1,2})
[...]

This should return SVector(NaN,NaN), since var(Float64[]) === NaN.