fix: handle private profiles and empty repositories during badge gene…

[anshika1179]

Description

Fixes badge generation failures (HTTP 500) for GitHub users with private profiles, no public repositories, or no visible contribution data.

I've added null-safe data handling for GraphQL responses in lib/github.ts and defensive validation across lib/calculate.ts so the system smoothly returns an empty-state badge instead of crashing. All 1,143 unit tests including the new edge-case scenarios passed successfully.

Fixes #1902

Pillar

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

Visual Preview

(No visual changes to standard badges; empty users will now correctly render an empty state badge instead of crashing.)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

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[github-actions]

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