Fix runtime error in a broken clock challenge caused by missing element check

[soralvy]

What’s the issue?

The Clock challenge component throws a runtime error because it tries to set the className on an element that does not yet exist in the DOM. Specifically, document.getElementById('time') returns null during the initial render, causing this error:

This PR adds a simple existence check before accessing the element’s className, preventing the runtime error while preserving the original logic

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[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

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[rickhanlonii]

@gaearon is the error intentional for the challenge? The text sounds like it shouldn't, but looking at the history I don't see how this would have ever worked without an error?