Dimension of SOS sets

Should we forbid dimension=0 for SOS sets? What does this even mean?

You could argue that throwing an error on `MOI.SOS1(Float64[])` is a breaking change, but I don't really set a good reason why anyone would ever want to do it. There's also an argument for making `dimension > 1` for SOS1, and `dimension > 2` for SOS2...

```Julia
julia> using JuMP

julia> model = Model();

julia> @constraint(model, VariableRef[] in SOS1())
[] ∈ MathOptInterface.SOS1{Float64}(Float64[])
```

Discuss...

Start with the MOI.SOS1(Float64[]) example and the SOS1/SOS2 dimension questions in the issue. Trace how these sets are represented and constructed, then clarify the intended validity rules and add coverage showing the resulting behavior.

索引モデルが issue の本文から書いたものです。