Allow apply_simple_constraint! with nothing

src/variables.jl

@@ -423,6 +423,8 @@ Add a single constraint or objective to the `variable`.
 - `equation`: fix variable to the result of this equation
 """
 apply_simple_constraint!(variable::AbstractVector, value::Symbol) = apply_simple_constraint!(sum(variable), Val(value))
+apply_simple_constraint!(variable, value::Nothing) = nothing
+apply_simple_constraint!(variable::NamedTuple, value::Nothing) = nothing
 apply_simple_constraint!(variable::VariableType, value::Symbol) = apply_simple_constraint!(variable, Val(value))
 apply_simple_constraint!(variable::VariableType, ::Val{:min}) = @objective global_model() Min objective_function(global_model()) + variable
 apply_simple_constraint!(variable::VariableType, ::Val{:max}) = @objective global_model() Min objective_function(global_model()) - variable