fix(calculate): return 'N/A' instead of empty string for mostActiveDate on empty calendar

[ChetanSenta]

Description

Fixes #3642

Pillar

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

Visual Preview

No SVG visual change. This is a pure data correctness fix in lib/calculate.ts.
GithubWrapped.tsx will now render "N/A" instead of a blank field for users
with zero contributions.

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 starred 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 server for faster collaboration, mentorship, and PR support.

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[Aamod007]

LGTM! The fix for mostActiveDate empty string is correct.