new DynamoDBQueryExpression<XXX>()
            .withHashKeyValues(YYY).withKeyConditionExpression(GSI != NULL);

java.lang.IllegalArgumentException: Input value must not be null

com.amazonaws.AmazonServiceException: One or more parameter values were invalid: An AttributeValue may not contain an empty string

Map<String, AttributeValue> eav = new HashMap<String, AttributeValue>();
eav.put(":val1", new AttributeValue().withS("Some value"));

DynamoDBQueryExpression<XXX> queryExpression = new DynamoDBQueryExpression<XXX>();
    queryExpression.withHashKeyValues(hashKeyValues);
    queryExpression.withFilterExpression("docType <> :val1").withExpressionAttributeValues(eav);