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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_prep_value(self, value):
 """Before encryption, need to prepare values."""

        value = super(NullBooleanPGPPublicKeyField, self).get_prep_value(value)
        if value is None:
            return None
        return "%s" % bool(value)