At the hearing, the court, on motion of the minor or on its own motion, shall order that the petition be dismissed and that the minor be discharged from any detention or restriction therefore ordered, after the presentation of evidence on behalf of the petitioner has been closed, if the court, upon weighing the evidence then before it, finds that the minor is not a person described by Section 601 or 602. If such a motion at the close of evidence offered by the petitioner is not granted, the minor may offer evidence without first having reserved that right.