fix: handle None values in hybrid result deduplication

- Use (r.get('url') or '') instead of r.get('url', '')
- Prevents 'NoneType' object has no attribute 'lower' error
- Allows hybrid search results to be properly merged and ranked

src/core/ranking/hybrid_result_ranker.py

@@ -203,8 +203,8 @@ class HybridResultRanker:
         unique = []
         
         for r in results:
-            url = r.get("url", "").lower().strip()
-            title = r.get("title", "").lower().strip()
+            url = (r.get("url") or "").lower().strip()
+            title = (r.get("title") or "").lower().strip()
             
             # Generate URL hash for exact matching
             url_hash = hashlib.md5(url.encode()).hexdigest() if url else None