Part of #63832

* Manually added strict null checks in around 70 files

* Fixed tsconfig.strictNullChecks.json merge fail

* Touched up a few unnecessary type changes

* Removed files already in other PRs

- False now really means false
- The default is true for all platforms

Fixes #65037

Part of #65035

This reverts commit dbdbbe52.

Part of #60565

Fixes #58015