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.

Written by the indexing model from the issue text.