Return type after querying attributes of empty vectors

[odow]

I don't know what we should do, but it isn't this:

julia> import MathOptInterface as MOI

julia> m = MOI.Utilities.Model{Float64}()
MOIU.Model{Float64}

julia> x = MOI.VariableIndex[]
MathOptInterface.VariableIndex[]

julia> MOI.get(m, MOI.VariablePrimalStart(), x)
Union{}[]