Enhance empty fallback tests for GitHub contributions

[rudrakshtank]

Add tests for handling missing user parameter and GitHub API errors.

Description

Fixes #8108

Pillar

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

Visual Preview

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.

[github-actions]

👋 Hey @rudrakshtank!

Unfortunately, this PR has been automatically closed because the referenced issue #8108 does not exist (or was deleted).

To resolve this, please do the following:

1. Link a valid open issue by editing your PR description to include a closing keyword (e.g., Fixes #<issue-number>).
2. Reopen this PR once the link is added.

Please link a valid open issue before this PR can be reviewed. 🙏