fix: allow the asset directory to be omitted in Wrangler config for commands that don't need it

[petebacondarwin]

Fixes #0000

We want to allow build tools (e.g. Nuxt/Vite/Next.js etc) to own the main and assets.directory fields in deployment configuration since they are the ones that generate these files.
In that case it doesn't makes sense for the user to provide them in their user configuration.
The main field is already optional from the point of view of the syntax check of the user's configuration file (e.g. wrangler.toml); this PR just makes the assets.directory field the same.
It is still an error to attempt to deploy or dev a Worker with an assets field but no assets.directory.

---

• Tests
  • TODO (before merge)
  • Tests included
  • Tests not necessary because:
• E2E Tests CI Job required? (Use "e2e" label or ask maintainer to run separately)
  • I don't know
  • Required
  • Not required because:
• Public documentation
  • TODO (before merge)
  • Cloudflare docs PR(s):
  • Documentation not necessary because: the docs don't specify that the wrangler.toml must have a directory, just that a directory is required.

[changeset-bot]

🦋 Changeset detected

Latest commit: e4413c9

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

This PR includes changesets to release 2 packages

Name	Type
wrangler	Patch
@cloudflare/vitest-pool-workers	Patch

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[github-actions]

A wrangler prerelease is available for testing. You can install this latest build in your project with:

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npm install https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/12155292375/npm-package-cloudflare-vitest-pool-workers-7426

npm install https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/12155292375/npm-package-cloudflare-workers-editor-shared-7426

npm install https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/12155292375/npm-package-cloudflare-workers-shared-7426

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---

wrangler@3.92.0 includes the following runtime dependencies:

Package	Constraint	Resolved
miniflare	workspace:*	3.20241106.2
workerd	1.20241106.2	1.20241106.2
workerd --version	1.20241106.2	2024-11-06

Please ensure constraints are pinned, and miniflare/workerd minor versions match.

[penalosa]

LGTM, but could you add some context to the PR description for why this is needed?