types: support stricter typechecking for returnEmptyString and returnNull

[hsource]

This is an adaptation of #2000 addressing the feedback and rebasing.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Motivation

For TypeScript users who are using key fallback (returnEmptyString or returnNull set to false), the improved type inference introduced in v23 isn't working at the moment. This PR fixes that.

Fix

Check the returnEmptyString and returnNull settings when deciding whether we can return null/empty string or fallback to the key.

Testing

Added tests; ran on a large codebase and verified it typechecked still

[coveralls]

coverage: 96.214%. remained the same
when pulling e358e89 on wanderlog:harry-key-fallback
into eb1207c on i18next:master.

[adrai]

thank you....
included in v23.8.1

[hsource]

Thanks for the fast pulls!