refactor(github): handle null GitHub names safely (JhaSourav07#483)

## Description

Fixes JhaSourav07#388

This PR addresses the issue by handling cases where a GitHub user has a
null or empty string name.
- Extracts `displayName` helper function to encapsulate fallback logic.
- Uses proper TypeScript type guards (`typeof profile.name ===
'string'`) and `trim()` to handle valid names, empty strings, and null
names gracefully.
- Replaces inline `||` fallback behavior with the helper usage.
- Fallback behavior (to `profile.login`) and other behaviors remain
unchanged without any regressions.

## Pillar

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

## Visual Preview

<img width="751" height="467" alt="2026-05-25_23-28-18"
src="https://github.com/user-attachments/assets/92f51b80-802a-4ecd-b66e-551cd7d21fb1"
/>


## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] 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).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/github.ts

@@ -122,6 +122,13 @@ export function validateGitHubUsername(username: string): boolean {
   return /^[a-z\d](?:[a-z\d]|-(?=[a-z\d])){0,38}$/i.test(username);
 }
 
+export function displayName(profile: GitHubUserProfile): string {
+  if (typeof profile.name === 'string' && profile.name.trim() !== '') {
+    return profile.name;
+  }
+  return profile.login;
+}
+
 export async function fetchGitHubContributions(
   username: string,
   options: FetchOptions = {}
@@ -355,7 +362,7 @@ export async function getFullDashboardData(username: string, options: FetchOptio
     // 1. Profile Mapping
     const profile = {
       username: profileData.login,
-      name: profileData.name || profileData.login,
+      name: displayName(profileData),
       avatarUrl: profileData.avatar_url,
       isPro: profileData.plan?.name === 'pro',
       bio: profileData.bio || 'No bio available',