fix: check vardef.regions instead of vardef in transfer_variables_or_parameters! (#213)

`!isnothing(vardef)` trivially always true; should be `!isnothing(vardef.regions)`.
When a variable has `regions = nothing` (single-region model default), the old
condition entered the branch and called `empty!(nothing)`, crashing with a
MethodError. This blocked the `setup_reservoir_model(reservoir, template; wells=...)`
overload from working on any standard single-region model.

Author: ancorso

src/utils.jl

@@ -2798,7 +2798,7 @@ function transfer_variables_or_parameters!(vars, new_model::SimulationModel, rep
         end
         Jutul.delete_variable!(new_model, varname)
         vardef = deepcopy(vardef)
-        if hasproperty(vardef, :regions) && !isnothing(vardef)
+        if hasproperty(vardef, :regions) && !isnothing(vardef.regions)
             entity = Jutul.associated_entity(vardef)
             n = count_entities(new_model.domain.representation, entity)
             empty!(vardef.regions)