test(svg): verify null deltaPercentage renders as N/A in generateMonthlySVG (JhaSourav07#2226)

## Description

Fixes JhaSourav07#1047

## Pillar

- [x] 📐 Pillar 2 — Geometric SVG Improvement


## Visual Preview
This adds integration tests to assert `generateMonthlySVG` rendering
behavior when `deltaPercentage` is `null` (e.g. when previous month's
commits are zero). It verifies `'N/A'` is rendered for both `percent`
and `both` delta formats.

## 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 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/svg/generator.test.ts

@@ -1066,6 +1066,41 @@ describe('generateMonthlySVG', () => {
 
     expect(svg).toContain('COMMITS THIS MONTH');
   });
+
+  it('renders monthly stats correctly with null deltaPercentage for delta_format percent', () => {
+    const nullDeltaStats: MonthlyStats = {
+      currentMonthTotal: 15,
+      previousMonthTotal: 0,
+      deltaPercentage: null,
+      deltaAbsolute: 15,
+      currentMonthName: 'June',
+    };
+
+    const svg = generateMonthlySVG(nullDeltaStats, {
+      user: 'octocat',
+      delta_format: 'percent',
+    } as unknown as BadgeParams);
+
+    expect(svg).toContain('N/A');
+    expect(svg).not.toContain('%');
+  });
+
+  it('renders monthly stats correctly with null deltaPercentage for delta_format both', () => {
+    const nullDeltaStats: MonthlyStats = {
+      currentMonthTotal: 15,
+      previousMonthTotal: 0,
+      deltaPercentage: null,
+      deltaAbsolute: 15,
+      currentMonthName: 'June',
+    };
+
+    const svg = generateMonthlySVG(nullDeltaStats, {
+      user: 'octocat',
+      delta_format: 'both',
+    } as unknown as BadgeParams);
+
+    expect(svg).toContain('N/A (+15)');
+  });
 });
 
 describe('shading and gradients', () => {