fix: Return empty array instead of null for query_coords when no results

When vector visualization search returns zero results, the code was returning
query_coords: null, which caused JavaScript error "can't access property 0,
queryCoords is null" when the frontend tried to access the array.

Changed to return empty array [] to match expected type and prevent crash.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/auth/viz_routes.py

@@ -212,7 +212,7 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                     "success": True,
                     "results": [],
                     "coordinates_3d": [],
-                    "query_coords": None,
+                    "query_coords": [],
                     "message": "No results found",
                 }
             )