Fix type errors in Vite example app

[zhougsoft]

hello! I was getting some minor TypeScript red squigglies in the Vite demo app that I was able to resolve with these changes. TS was complaining about accessing possible null refs (video & message) and data.buffer.

Since ffmpeg.readFile returns type FileData, seemed that the data needed to be cast as ArrayBuffer then converted to Uint8Array to access the buffer property.

I didn't see any contribution guidelines for the examples/misc stuff, sorry if this isn't how it's done 😅

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[jeromewu]

Looks great! Thanks for the PR.