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/server/api/endpoints/gallery/posts/update.ts

@@ -10,6 +10,7 @@ import type { DriveFilesRepository, GalleryPostsRepository } from '@/models/_.js
 import type { MiDriveFile } from '@/models/DriveFile.js';
 import { GalleryPostEntityService } from '@/core/entities/GalleryPostEntityService.js';
 import { DI } from '@/di-symbols.js';
+import { isNotNull } from '@/misc/is-not-null.js';
 
 export const meta = {
 	tags: ['gallery'],
@@ -67,7 +68,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					id: fileId,
 					userId: me.id,
 				}),
-			))).filter((file): file is MiDriveFile => file != null);
+			))).filter(isNotNull);
 
 			if (files.length === 0) {
 				throw new Error();