fix: correctly deserialize on the server

[eltigerchino]

fixes #13675

This PR adds an optional chain for when deserialize tries to access app.decoders since app is always undefined on the server and is only initialised on the client.

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot]

🦋 Changeset detected

Latest commit: f9b2bd8

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13686-svelte.vercel.app/

[dummdidumm]

While this works, I'm wondering if this could introduce inconsistencies since you might expect that the decoders you defined will be used, but on the server they don't exist, i.e. will be ignored.

[eltigerchino]

I've updated deserialize to use the decoders defined by the user on the server too.

[dummdidumm]

Nice!