chore: fix consistent-return eslint warnings

[kieckhafer]

Resolves #5272
Impact: breaking|minor
Type: chore

Issue

We currently have many consistent-return eslint warnings.

Solution

Fix (or disable) these warnings, and then remove the warn override in our package.json eslint rules to make consistent-return an error so new violations are not introduced.

Most of the changes add null as a return. Some return a boolean where deemed appropriate.

Breaking changes

return null has been added in a lot of places. While this shouldn't affect anything that is updated, please not that there is no longer no return / undefined return in many functions.

Testing

1. run eslint
2. see there are no longer any consistent-return warnings
3. see that the override for this rule was removed, so in the future all issues will be errors

Notes

• There are a few jsdocs added in the PR as well, as I tried to add them if I saw missing jsdoc in a file I was updating.

[aldeed]

One problem and one suggestion. The rest looks good.

@aldeed comments addressed, ready for another look