fix: undefined property in unmatched host

[laverdet]

allowedMethods() throws a runtime error when the host of its router fails to match. Additionally it would throw if you never added routes() to your koa middleware chain.

Example:

import Router from "@koa/router";
import Koa from "koa";

const app = new Koa();
const router = new Router({ host: "aaaa" });
app.use(router.routes());
app.use(router.allowedMethods());
app.listen(8888);

$ curl localhost:8888/                   
Internal Server Error

Error:

  TypeError: Cannot read properties of undefined (reading 'length')
      at /Users/marcel/braid/web/.pnpm/@koa+router@12.0.1/node_modules/@koa/router/lib/router.js:487:41
      at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.