Handle empty content in sample sites

[exacs]

This PR fixes the sample sites after the change in #124.

• The hello-world site shows a 404 if content is not found
• In fx-variation, it shows the original if a variation is not found; if that original is also not found, shows a 404

[Copilot]

Pull Request Overview

This PR enhances error handling in sample sites to properly manage cases where content is not found, addressing issues introduced by changes in PR #124.

• Adds 404 handling for missing content in both sample sites
• Implements fallback logic in fx-integration to show original content when variations are not found
• Replaces error-based fallback with explicit empty content checks

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
samples/hello-world/src/app/[...slug]/page.tsx	Adds 404 handling when no content is found for the requested path
samples/fx-integration/src/app/en/[...slug]/page.tsx	Implements fallback logic and 404 handling with a reusable helper function

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