DynamoDB enhanced doesn't create a new item on UpdateItem with null values and ignoreNulls=false

If I want to create a new item with UpdateItem and some of my attributes are null then the item is not created. But if I use ignoreNulls=true then everything works.

## Describe the bug

I would expect that the item is always created regardless of the ignoreNull parameter. PutItem works as expected.

## Expected Behavior

A new item should always been created with UpdateItem.

## Current Behavior

A new item is only created with UpdateItem if all attributes are set not non-null values or if some attributes are null then ignoreNulls has to be set to true.

## Steps to Reproduce

## Possible Solution

## Context

## Your Environment

* AWS Java SDK version used:
* JDK version used:
* Operating System and version: