server: fix user deletes being stuck in queue

The user was already deleted from the user cache, so requesting the
user returned null. Because the key was not null, there was a non-null
return, in turn making further code think, fetching the user was
successful.

packages/backend/src/remote/activitypub/misc/auth-user.ts

@@ -35,10 +35,9 @@ export async function authUserFromKeyId(keyId: string): Promise<AuthUser | null>
 	return await publicKeyCache.fetch(keyId)
 		.then(async key => {
 			if (!key) return null;
-			else return {
-				user: await userByIdCache.fetch(key.userId),
-				key,
-			};
+			const user = await userByIdCache.fetch(key.userId);
+			if (!user) return null;
+			return { user, key };
 		});
 }