refactor: use isDefined and isUndefined consistently

[lucastetreault]

Fix any place that x == undefined or x != undefined
to use isUndefined and isDefined instead

Closes #4365

[lgalfaso]

Hi, thanks for the PR. Can you please

• Squash everything into a single commit
• Fix the commit comment as this is not changing x == undefined and x != undefined as it is changing x === undefined and x !== undefined (that have a slightly different meaning)

[petebacondarwin]

@lgalfaso thanks for triaging this one. I think that if it looks good then we can do the squash and commit message fix up when merging to save on cycles.

Thanks @lucastetreault for fixing this very long running issue!