test(cache): verify TTLCache behavior for empty string keys (Variation 2) (JhaSourav07#1402) (JhaSourav07#2197)

## Description

Fixes JhaSourav07#1402 

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

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(`localhost:3000/api/streak?user=YOUR_USERNAME`).
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---

I have tested every tests for these changes. 

<img width="997" height="126" alt="Screenshot 2026-06-01 020511"
src="https://github.com/user-attachments/assets/598e9b04-a4bb-486f-b219-ec2ab55ffd85"
/>

Author: JhaSourav07

lib/cache.test.ts

@@ -504,6 +504,34 @@ describe('TTLCache', () => {
       cache.destroy();
     });
 
+    it('verify TTLCache behavior for empty string keys (Variation 2)', () => {
+      const cache = new TTLCache<string>();
+
+      // Assert that setting a value with empty string key throws error
+      expect(() => {
+        cache.set('', 'test-value', 60_000);
+      }).toThrow(Error);
+
+      // Verify the error message is correct
+      expect(() => {
+        cache.set('', 'test-value', 60_000);
+      }).toThrow('Cache key cannot be empty');
+
+      // Verify that cache remains empty (no entry for empty key)
+      expect(cache.has('')).toBe(false);
+      expect(cache.get('')).toBeNull();
+
+      // Verify cache size is still 0
+      expect(cache.size()).toBe(0);
+
+      // Verify that normal operations still work after failed attempt
+      cache.set('valid-key', 'value', 60_000);
+      expect(cache.get('valid-key')).toBe('value');
+      expect(cache.size()).toBe(1);
+
+      cache.destroy();
+    });
+
     it('handles rapid get/set/delete cycles', () => {
       const cache = new TTLCache<number>();
       for (let i = 0; i < 100; i++) {