[DynamoDB] Empty values for string are now supported

[hassankhan]

Relevant AWS blog post: https://aws.amazon.com/about-aws/whats-new/2020/05/amazon-dynamodb-now-supports-empty-values-for-non-key-string-and-binary-attributes-in-dynamodb-tables/

Happy to create a PR, was just wondering if the old text should completely be replaced, or if another line should be added after the existing bulletpoint?

[bschaatsbergen]

Do what fits you and think is best and easiest to maintain on long term.

Incase the whole section is outdated, replace it or just re-format it properly.

Thanks for taking a look!