fix: only change objective sense if given

Author: rschwarz

mip/highs.py

@@ -436,7 +436,8 @@ def set_objective(self: "SolverHighs", lin_expr: "mip.LinExpr", sense: str = "")
             status = self._lib.Highs_changeColCost(self._model, var.idx, coef)
 
         self.set_objective_const(lin_expr.const)
-        self.set_objective_sense(lin_expr.sense)
+        if lin_expr.sense:
+            self.set_objective_sense(lin_expr.sense)
 
     def set_objective_const(self: "SolverHighs", const: numbers.Real):
         status = self._lib.Highs_changeObjectiveOffset(self._model, const)