fix(live-preview): skip stackblitz source fetch when no example is set

[chintankavathia]

Problem

Navigating to the live-preview app root (localhost:4200, which redirects to viewer/editor) produced a runtime console error:

GET http://localhost:4200/app/examples/undefined.ts 404 (Not Found)

Cause

The stackblitz button directive's httpResource built its request URL unconditionally:

private readonly exampleTs = httpResource.text(() => `${this.baseUrl()}${this.example()}.ts`);

When no example was selected, example() is undefined, so the resource fetched app/examples/undefined.ts, returning a 404.

Fix

Return undefined from the request factory when no example is set, which keeps the httpResource idle so no request is made until a real example is selected.

The button remains disabled when there is no example (already handled by !this.example() in the disabled computed).

---

Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request updates the SiStackblitzButtonDirective to conditionally fetch the TypeScript example file only when the example is defined, preventing unnecessary HTTP requests when the example value is falsy. I have no feedback to provide as there are no review comments.

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