Conform more of the codebase to strictNullChecks (#10350

* Conform more of the codebase to `strictNullChecks`

* Iterate

* Generics ftw

* Iterate

src/@types/common.ts

@@ -52,3 +52,5 @@ export type KeysStartingWith<Input extends object, Str extends string> = {
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
     [P in keyof Input]: P extends `${Str}${infer _X}` ? P : never; // we don't use _X
 }[keyof Input];
+
+export type NonEmptyArray<T> = [T, ...T[]];