fix: properly page through undefs and nulls

[jsalvata]

Changes Made

• Ensured that if the paginatedField's value is null or undefined, it gets properly transferred via the paging handles next and previous.
• If sorting case-insensitively, replace those with the empty string for comparing, as mongodb's $toLower transforms both null and undefined into it.
• If sorting case-sensitively, handle each of the cases, taking into account that undefined sorts before null and null before everything else.
• Change the README to instruct users to avoid mixing types (before it was avoiding emtpy values).
• Updated bson and mongoist even though it was not necessary for this ticket.

Potential Risks

It is unclear whether the conditions used to check for undefined and null (or non-undefined, non-null, and combinations) will use indices efficiently. But this only happens in the specific case where this change kicks in: it will not alter the performance of existing correct code.

Test Plan

See unit tests.

Checklist

• [x] I've increased test coverage
• [x] Since this is a public repository, I've checked I'm not publishing private data in the code, commit comments, or this PR.

[mixmax-bot]

:tada: This PR is included in version 8.1.1 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: