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/core/entities/DriveFileEntityService.ts

@@ -259,7 +259,7 @@ export class DriveFileEntityService {
 		options?: PackOptions,
 	): Promise<Packed<'DriveFile'>[]> {
 		const items = await Promise.all(files.map(f => this.packNullable(f, options)));
-		return items.filter((x): x is Packed<'DriveFile'> => x != null);
+		return items.filter(isNotNull);
 	}
 
 	@bindThis