fix(mdxish): properly parse embed magic blocks

[eaglethrost]

	Fix RM-XYZ

🧰 Changes

There were 2 bugs regarding [block:embed] magic blocks in mdxish:

• It could crash the page
• Even if they don't it wasn't being parsed correctly

Fixes:

• When parsing embed magic blocks & setting the children, don't send in null as fallback, because it can create a type error later on. Use an empty string instead
• Create an embed handler to map the embed node to Embed component

🧬 QA & Testing

• Broken on production.
• Working in this PR app.