fix: only send durable object migrations when required

[threepointone]

We had a bug where even if you'd published a script with migrations, we would still send a blank set of migrations on the next round. The api doesn't accept this, so the fix is to not do so. I also expanded test coverage for migrations.

Fixes #705

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This is the first in a set of a few PRs I'll send for durable objects. Next, more validations based on exports.

[changeset-bot]

🦋 Changeset detected

Latest commit: 47ed919

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

This PR includes changesets to release 1 package

Name	Type
wrangler	Patch

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[github-actions]

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

npm install --save-dev https://prerelease-registry.developers.workers.dev/runs/2071558805/npm-package-wrangler-728

You can reference the automatically updated head of this PR with:

npm install --save-dev https://prerelease-registry.developers.workers.dev/prs/728/npm-package-wrangler-728

Or you can use npx with this latest build directly:

npx https://prerelease-registry.developers.workers.dev/runs/2071558805/npm-package-wrangler-728 dev path/to/script.js

[threepointone]

This is a warning and not an error because folks may have purposely deleted the [migrations] section from their wrangler.toml altogether. That's not great, but it's not something we can prevent.

[threepointone]

I'm going to remove the exports validation from this PR and send it in the next one

[threepointone]

Ok done, this is ready for review.

[petebacondarwin]

Ship it!