[FLINK-36428][Connectors/DynamoDB] DynamoDb Table API Sink fails when null value in the RowData

[maoxingda]

Purpose of the change

Add an option ignore-nulls to determine whether to ignore null values, instead of hardcoding it into the code.

Verifying this change

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Significant changes

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[boring-cyborg]

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[maoxingda]

Could someone help review this pull request?

[hlteoh37]

LGTM, thanks for spotting and fixing, @maoxingda ! Can you please update the docs as well please!

English:
https://github.com/apache/flink-connector-aws/blob/main/docs/content/docs/connectors/table/dynamodb.md

Chinese:
https://github.com/apache/flink-connector-aws/blob/main/docs/content.zh/docs/connectors/table/dynamodb.md