fix: explicitly check firstError !== null in getGraphQLErrorMessage (JhaSourav07#1871)

## Description

Fixes JhaSourav07#1850 

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [X] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
No visual changes, this is a runtime safety fix in lib/github.ts.

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- [X] I have read the `CONTRIBUTING.md` file.
- [ ] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [X] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
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`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
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- [X] I have made sure that i have only one commit to merge in this PR.
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Author: JhaSourav07

lib/github.ts

@@ -173,7 +173,7 @@ function getGraphQLErrorMessage(errors: unknown): string {
   if (!Array.isArray(errors)) return 'GitHub GraphQL API returned an unknown error';
   const firstError = errors[0];
   if (
-    firstError &&
+    firstError !== null &&
     typeof firstError === 'object' &&
     'message' in firstError &&
     typeof firstError.message === 'string'