feature: Make the secondary index range key nullable

[ShauniArima]

As said in #103, Tempest does not allow using an empty sort key as secondary index.

I do not know how to test it correctly. This part of the code seems hard to test. Does someone have some tips for me?

[kyeotic]

This looks good, thanks for taking the time to put this together. I need to dig into this to make sure its not going to cause any other issues, but I think I should be able to accept it. I should have some time tomorrow or friday to do that.

[zhxnlai]

Thanks for the contribution. You may also want to update that code that integrates with AWS SDK V1: it currently checks for requireNotNull on global secondary index sort key. https://github.com/cashapp/tempest/blob/main/tempest/src/main/kotlin/app/cash/tempest/internal/V1.kt#L85

[ShauniArima]

Thanks for the contribution. You may also want to update that code that integrates with AWS SDK V1: it currently checks for requireNotNull on global secondary index sort key. https://github.com/cashapp/tempest/blob/main/tempest/src/main/kotlin/app/cash/tempest/internal/V1.kt#L85

Thanks!
I have updated the code for AWS SDK V1 too 😄