test: fix the Suspect sources tests CI rejected (#5410)

- staticcheck QF1001: the nil-vs-empty check in TestSourceMarkersFor is
  now an early continue instead of a negated conjunction.
- The page test mocks #lib/api/admin.js whole, so suspectSourcesNextOffset
  was undefined there. Its test moves to admin.suspect-sources.test.ts,
  beside admin.scan.test.ts, against the real module.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/routes/admin/suspect-sources/suspect-sources.test.ts

@@ -9,7 +9,6 @@ vi.mock('#lib/api/admin.js', () => ({
 
 import SuspectSourcesPage from './+page.svelte';
 import { createSuspectSourcesQuery } from '#lib/api/admin.js';
-import { suspectSourcesNextOffset } from '#lib/api/admin.js';
 
 const mocked = createSuspectSourcesQuery as ReturnType<typeof vi.fn>;
 
@@ -112,14 +111,3 @@ describe('Suspect sources page', () => {
     expect(screen.queryByRole('button', { name: /more/i })).not.toBeInTheDocument();
   });
 });
-
-describe('suspectSourcesNextOffset', () => {
-  test('counts tracks, not groups, and stops at the total', () => {
-    const groups = [
-      { directory: 'a', tracks: [track('1', '1.mp3', []), track('2', '2.mp3', [])] },
-      { directory: 'b', tracks: [track('3', '3.mp3', [])] }
-    ];
-    expect(suspectSourcesNextOffset(page(0, 10, groups))).toBe(3);
-    expect(suspectSourcesNextOffset(page(7, 10, groups))).toBeUndefined();
-  });
-});