MOI.is_valid is wrong for parameter constraints

[odow]

Found by @metab0t

julia> import Ipopt

julia> const MOI = Ipopt.MOI
MathOptInterface

julia> model = Ipopt.Optimizer();

julia> _, ci = MOI.add_constrained_variable(model, MOI.Parameter(1.0))
(MOI.VariableIndex(67553994410557440), MathOptInterface.ConstraintIndex{MathOptInterface.VariableIndex, MathOptInterface.Parameter{Float64}}(67553994410557440))

julia> MOI.is_valid(model, ci)
false