refactor(backend): 存在確認のfindOneByをexistに置き換え (#11224)

* refactor(backend): 存在確認の`findOneBy`を`exist`に置き換え

* cleanup

packages/backend/src/server/api/endpoints/following/delete.ts

@@ -84,12 +84,14 @@ export default class extends Endpoint<typeof meta, typeof paramDef> {
 			});
 
 			// Check not following
-			const exist = await this.followingsRepository.findOneBy({
-				followerId: follower.id,
-				followeeId: followee.id,
+			const exist = await this.followingsRepository.exist({
+				where: {
+					followerId: follower.id,
+					followeeId: followee.id,
+				},
 			});
 
-			if (exist == null) {
+			if (!exist) {
 				throw new ApiError(meta.errors.notFollowing);
 			}