public void setWhenMissingSetter(WhenMissing whenMissing) {
        this.whenMissing = Objects.requireNonNull(whenMissing);
    }
Sets the behavior of the deserializer when a setting is missing.

@param whenMissing Behavior when a property is not matched to a setter.
smithy-model/src/main/java/software/amazon/smithy/model/node/NodeMapper.java