Use `strictNullChecks` in `matrix-react-sdk`

[novocaine]

Your use case

https://www.typescriptlang.org/tsconfig#strictNullChecks

This will reduce the number of runtime errors caused by unexpected null and undefined values by disallowing them as a value unless explicitly permitted.

Related: matrix-org/matrix-js-sdk#2112

### Tasks
- [ ] https://github.com/vector-im/element-web/issues/24659
- [x] https://github.com/vector-im/element-web/issues/24658
- [ ] https://github.com/vector-im/element-web/issues/24657
- [ ] https://github.com/vector-im/element-web/issues/24656
- [x] https://github.com/vector-im/element-web/issues/24655
- [ ] https://github.com/vector-im/element-web/issues/24654
- [x] https://github.com/vector-im/element-web/issues/24653
- [x] https://github.com/vector-im/element-web/issues/24652
- [ ] https://github.com/vector-im/element-web/issues/24651
- [ ] https://github.com/vector-im/element-web/issues/24650
- [x] https://github.com/vector-im/element-web/issues/24649
- [ ] https://github.com/vector-im/element-web/issues/24756
- [ ] https://github.com/vector-im/element-web/issues/24755
- [ ] https://github.com/vector-im/element-web/issues/24754
- [ ] https://github.com/vector-im/element-web/issues/24753
- [ ] https://github.com/vector-im/element-web/issues/24822
- [ ] https://github.com/vector-im/element-web/issues/24836
- [ ] https://github.com/vector-im/element-web/issues/24935
- [ ] https://github.com/vector-im/element-web/issues/24936
- [ ] https://github.com/vector-im/element-web/issues/24938
- [ ] https://github.com/vector-im/element-web/issues/24937
- [ ] https://github.com/vector-im/element-web/issues/24940
- [ ] https://github.com/vector-im/element-web/issues/24939
- [ ] https://github.com/vector-im/element-web/issues/24941
- [ ] https://github.com/vector-im/element-web/issues/25217
- [ ] https://github.com/vector-im/element-web/issues/24944
- [ ] https://github.com/vector-im/element-web/issues/24945
- [ ] https://github.com/vector-im/element-web/issues/24946
- [ ] https://github.com/vector-im/element-web/issues/24948
- [ ] https://github.com/vector-im/element-web/issues/25017
- [ ] https://github.com/vector-im/element-web/issues/25086
- [ ] https://github.com/vector-im/element-web/issues/24943
- [ ] https://github.com/vector-im/element-web/issues/25087
- [ ] https://github.com/vector-im/element-web/issues/25088
- [ ] https://github.com/vector-im/element-web/issues/25370
- [ ] https://github.com/vector-im/element-web/issues/24752
- [ ] https://github.com/vector-im/element-web/issues/24947
- [ ] https://github.com/vector-im/element-web/issues/24942
- [ ] https://github.com/vector-im/element-web/issues/24838
- [ ] https://github.com/vector-im/element-web/issues/25344
- [x] Apply `strictNullChecks` to `src/components/structures/InteractiveAuth.tsx`
- [x] Apply `strictNullChecks` to `src/Lifecycle.ts`
- [x] Apply `strictNullChecks` to `src/components/structures/RoomView.tsx`
- [x] Apply `strictNullChecks` to `src/components/structures/ScrollPanel.tsx`
- [x] Apply `strictNullChecks` to `src/components/structures/TimelinePanel.tsx`
- [x] Apply `strictNullChecks` to `src/IdentityAuthClient.tsx` & `src/AddThreepid.ts`
- [x] Apply `strictNullChecks` to `src/components/views/rooms/R*.tsx`
- [x] Apply `strictNullChecks` to `src/createRoom.ts`
- [x] Apply `strictNullChecks` to `src/components/views/rooms/[NPST]*.tsx`
- [x] Apply `strictNullChecks` to `src/components/views/dialogs/*`
- [x] Apply `strictNullChecks` to `src/components/structures/MatrixChat.tsx`
- [x] Apply `strictNullChecks` to `src/audio/*`
- [x] Apply `strictNullChecks` to `src/customisations/Media.ts`
- [ ] https://github.com/vector-im/element-web/issues/25345

Have you considered any alternatives?

No response

Additional context

No response

[MadLittleMods]

We currently have 5738 errors when enabling strictNullChecks on matrix-react-sdk.

It would be nice to start using this for all of our new code and only apply strictNullChecks to the changes in the PR (from the git diff). We can run Typescript against the files that changed and filter the errors/warnings to only the line numbers that changed.

Looks like there are some options for ESLint which we could base off of:

• https://www.npmjs.com/package/eslint-plugin-diff
• https://github.com/grvcoelho/lint-diff

For staged files:

• https://github.com/okonet/lint-staged
• https://github.com/hugoMencoboni/tslint-staged

---

Alternatively, we could add an extra assertion that the the number of strictNullChecks can't go up from the number on develop which means you have to fix any new code introduced.

[novocaine]

Thanks @MadLittleMods, that seems like a great approach.

If you're interested in helping working through that backlog .. we could use as many hands as we can get to chip away at it