Fix var expectation weights when auxiliary model has a constant.

Increment index variables_id_in_var if constant is present in VAR model (first position in the Companion representation)..

(cherry picked from commit d440b708)

matlab/+var_expectation/update_parameters.m

@@ -108,6 +108,10 @@ for i = 1:m
     end
 end
 
+if isfield(auxmodel, 'isconstant') && auxmodel.isconstant
+    variables_id_in_var = variables_id_in_var+1;
+end
+
 % Get the horizon parameter.
 horizon = varexpectationmodel.horizon;
 
@@ -214,4 +218,4 @@ if isequal(varexpectationmodel.auxiliary_model_type, 'var')
     end
 else
     DynareModel.params(varexpectationmodel.param_indices) = parameters;
-end
+end
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