Empty string values for key attributes

[yaronya]

Describe the bug This is more a regression than a bug. I've upgraded from 6.0.0 to 7.1.0 and I've noticed you started supporting empty values for strings but the latest change from AWS applies only to non-key attributes so I had a regression where I try to save a key as an empty string. I obviously changed that but this is confusing 🤔

To Reproduce

1. Set an attribute as secondary key.
2. Save it with empty string.
3. DDB will fail - secondary index cannot be an empty string.

Expected behavior There shouldn't be a regression. IMO you should persist old logic for key attributes, while only non-key attributes will be saved with an empty string.

[yaronya]

@michaelwittwer is there a plan to fix this bug in the near future? Is there a plan to have a release soon? If so I might contribute and fix it.