refactor(backend): Array.prototype.filterでの非null確認ではisNotNull関数を使うように (#13442)

* `Array.prototype.filter`での非null確認では`isNotNull`関数を使うように

* `{}` -> `NonNullable<unknown>`

packages/backend/src/misc/is-not-null.ts

@@ -3,8 +3,6 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-// we are using {} as "any non-nullish value" as expected
-// eslint-disable-next-line @typescript-eslint/ban-types
-export function isNotNull<T extends {}>(input: T | undefined | null): input is T {
+export function isNotNull<T extends NonNullable<unknown>>(input: T | undefined | null): input is T {
 	return input != null;
 }