add support for optional values in "where" clauses

[maybeanerd]

Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions? not needed
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)? potentially?
• Did you update the typescript typings accordingly (if applicable)?
• Did you follow the commit message conventions explained in CONTRIBUTING.md?

Description of change

fixes #11564
as discussed in the issue, this might not be the wanted solution. I'm opening a PR though because the issue has been dead for a while and as long as there is no real reason against this change, it will fix the issue.

[codecov]

Codecov Report

Merging #12337 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12337   +/-   ##
=======================================
  Coverage   96.40%   96.40%           
=======================================
  Files          95       95           
  Lines        9115     9115           
=======================================
  Hits         8787     8787           
  Misses        328      328           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 65a9e1e...ad02519. Read the comment docs.

[sushantdhiman]

Please add a test here https://github.com/sequelize/sequelize/blob/master/types/test/where.ts

[maybeanerd]

@sushantdhiman done, I hope I understood how the test is supposed to work 😁