Migrate to null-safety

[nyarian]

No description provided.

[Francessco121]

This is pretty good so far. I do have some suggestions and concerns that I added. Other than those, MSAL's source still needs to be looked at to see what properties can be safely non-nullable.

I apologize if this review seems a bit overkill or nitpicky, I'm just trying to keep the code consistent and safe. It's also my first time ever doing a code review on GitHub :). I'll probably tweak a few things myself later anyway, so don't worry too much about the nitpicks but I'd appreciate if you could change the ones I noted.

[nyarian]

I believe I've fixed the issues you outlined and resolved the appropriate discussions, but there are a few discussions that are not as straightforward so I'd ask you to resolve them if you'll see the fixes done as applicable.

once you are ready to merge this, please, ping me out so I'll squash the commits before that so the git won't contain trash commits like 'apply requested PR changes'

[Francessco121]

Thanks, yeah this is pretty good, I just have one issue with the changes to cache_location.

I would also still like to go through the source of msal.js to determine the nullability of various properties. If you're not comfortable with that, I can take over and finish this up.

How important is it to you that this package is null-safe? Is it blocking one of your projects? I wasn't planning on releasing a null-safe version for a while until your PR came along, so if it's an issue I can get this published soon.

[nyarian]

as much as I would like to sync msal-js properties with the ones in this library, I'm simply not qualified with either JS or TS, so I'd rather leave it for someone who is

the package is quite important for me to be null-safe, but I'm feeding from my own fork already instead of using the pub.dev version, so I have no issues with that. this contribution is just for karma points

[Francessco121]

Ok, no problem. I'll finish this up soon. Thanks a ton for getting it this far!