fix: fetchEvent -> HeaderProxy checks for null instead of undefined

[huseeiin]

PR Checklist

Please check if your PR fulfills the following requirements:

• Addresses an existing open issue: fixes #000
• Tests for the changes have been added (for bug fixes / features)

What is the current behavior?

event.response.headers.has("does-not-exist") returns true

What is the new behavior?

event.response.headers.has("does-not-exist") returns false

Other information

[netlify]

✅ Deploy Preview for solid-start-landing-page ready!

Name	Link
🔨 Latest commit	5d5c872
🔍 Latest deploy log	https://app.netlify.com/projects/solid-start-landing-page/deploys/68b0cf65c2b1f4000987eba8
😎 Deploy Preview	https://deploy-preview-1919--solid-start-landing-page.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[pkg-pr-new]

More templates

• example-bare
• example-basic
• example-experiments
• example-hackernews
• example-notes
• example-todomvc
• example-with-auth
• example-with-authjs
• example-with-drizzle
• example-with-mdx
• example-with-prisma
• example-with-solid-styled
• example-with-solidbase
• example-with-tailwindcss
• with-tanstack-router
• example-with-trpc
• example-with-unocss
• example-with-vitest

npm i https://pkg.pr.new/@solidjs/start@1919

commit: 5d5c872

[atilafassina]

thanks!! 🏆

[changeset-bot]

🦋 Changeset detected

Latest commit: 5d5c872

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

This PR includes changesets to release 1 package

Name	Type
@solidjs/start	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