rm strictNullChecks globally (#2973)

because already covered by `strict` by `@tsconfig/node20` base

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* Chores
* Updated TypeScript configuration by disabling strict null checks,
reducing nullability strictness during development and builds.
* Tooling-only adjustment to improve developer experience and
compatibility; no changes to public APIs.
* No user-facing impact: application behavior, features, performance,
and UI remain unchanged.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: RobinTail

tsconfig.json

@@ -4,7 +4,6 @@
     "noImplicitAny": true,
     "noImplicitOverride": true,
     "noUncheckedSideEffectImports": true,
-    "strictNullChecks": true,
     "types": ["vitest/globals"]
   }
 }