Technical debt: Enable null-checks in TypeScript

[olemartinorg]

Description

We currently have "strictNullChecks": false in our tsconfig.json, which lets us write dangerous code that never checks for types/properties that could be nullable/undefined. This issue alone has lead to multiple hard crashes/TypeErrors in production code this year alone. Setting this value to true causes 954 errors in the main branch right now, but cleaning these up is mostly a menial chore rather than something that requires acute attention to detail.

Prefer to do this at a point in time when there is less activity in the repo than usual.

Related issues

• Automatically generated end-to-end tests for apps app-template-dotnet#122
• An app owner should be able to pre-instantiate a form (with prefill) on behalf of the user altinn-studio#440
• Loading different versions of app-frontend-react in staging vs production app-lib-dotnet#109