Enable strictNullChecks for frontend and plugins

[sniok]

We should enable typescript strictNullChecks option to reduce number of issues that can be avoidable. Especially since we have a pretty good type coverage

Example bug that could have been avoided with strict null checks headlamp-k8s/plugins#151

This would also require us to fix all the existing issues surfaced by this option, so I expect a pretty big refactor to enable this
For plugins this will have to be opt-in

[DhairyaMajmudar]

This fix would be helpful in reducing the bugs. Would love to help in this refactor 🤝🏻

[Jineshbansal]

@sniok To resolve this issue, do I just need to add "strictNullChecks": true in the tsconfig.json file in the frontend directory and then fix any issues that come up during npm run build? Am I correct?

[sniok]

@Jineshbansal yeah, basically
enable strictNullChecks, then run npm run tsc. I'm expecting a lot of errors there so to keep it manageable we should fix only some of them at a time (per pull request) and then turn strictNullChecks off. Once all of them are fixed we can enable strictNullChecks for real to prevent any new issues from being added.