attributevalue marshal to AttributeValueMemberNULL with double pointer when outer pointer is not nil

[raduendv]

Fixes: attributevalue.MarshalMap() to AttributeValueMemberNULL field with double pointer #2726

Description

var nilVal *int
s := struct {
	Foo **int `dynamodbav:"foo,omitempty"`
}{
	Foo: &nilVal,
}

The property foo should be serialized to &types.AttributeValueMemberNULL{} instead of falling back to omitempty since the first level pointer is not nil.