fix(core): fix error when merging class attributes

[bdbch]

Please describe your changes

This PR fixes an issue that caused a crash in mergeAttributes when one of the value or mergedAttributes is not set.

How did you accomplish your changes

I added checks to the mergeAttribute function that will throw back a fallback value of [].

How have you tested your changes

I tried adding a class in my local demo. I also added a test to check, if the option is set correctly afterwards.

How can we verify your changes

Run the local demo and configure the extension to have a class – it should work without any error throws & the test should run through.

Checklist

• The changes are not breaking the editor
• Added tests where possible
• Followed the guidelines
• Fixed linting issues

[netlify]

✅ Deploy Preview for tiptap-embed ready!

Name	Link
🔨 Latest commit	b7f35ea
🔍 Latest deploy log	https://app.netlify.com/sites/tiptap-embed/deploys/64de428d4302290008359c56
😎 Deploy Preview	https://deploy-preview-4340--tiptap-embed.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 site configuration.