UpdateItemEnhancedRequest for null version number with `ignoreNulls(true)` results into ConditionalCheckFailedException

## Describe the issue
When we use `ignoreNulls(true)` feature while doing `table.updateItem` understanding that it will ignore all null fields to update it in db. But it looks like it's not working as expected for dynamo db version field, when my object contain all required fields like primary hash key, and sort key and version number as null, it's throwing `ConditionalCheckFailedException`.

## Steps to Reproduce
Where version number field is declared as below.
```
@Getter(onMethod_ = {@DynamoDbVersionAttribute})
private Long version;
```

```
customDynamoDbBeanObject.version(null);
dynamoDbTable.updateItem(UpdateItemEnhancedRequest.builder(CustomDynamoDbBean.class)
.item(customDynamoDbBeanObject)
.ignoreNulls(true)
.build());
```
## Current Behavior
It's throwing `ConditionalCheckFailedException`.

## Your Environment
* AWS Java SDK version used: AwsJavaSdk-DynamoDb-Enhanced = 2.0;
* JDK version used: JDK8 = 1.0;
* Operating System and version: Linux

Inizia da UpdateItemEnhancedRequest e table.updateItem in DynamoDB Enhanced Client, quindi traccia come viene gestito un @DynamoDbVersionAttribute null con ignoreNulls(true). Riproduci l'esempio CustomDynamoDbBean mostrato su JDK 8 con AWS SDK 2.0; il lavoro è completato quando la versione null viene ignorata senza ConditionalCheckFailedException e il comportamento dispone di una copertura di regressione.

Scritto dal modello di indicizzazione a partire dal testo della issue.