Fix NoneType error when asset provenance is null

Use 'or {}' pattern to handle both missing keys and explicit None
values for origin, provenance, and metadata fields.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server.py

@@ -743,9 +743,9 @@ async def ui_asset_detail(name: str, request: Request):
     asset_type = asset.get("asset_type", "")
     tags = asset.get("tags", [])
     description = asset.get("description", "")
-    origin = asset.get("origin", {})
-    provenance = asset.get("provenance", {})
-    metadata = asset.get("metadata", {})
+    origin = asset.get("origin") or {}
+    provenance = asset.get("provenance") or {}
+    metadata = asset.get("metadata") or {}
     created_at = asset.get("created_at", "")[:10]
 
     type_color = "bg-blue-600" if asset_type == "image" else "bg-purple-600" if asset_type == "video" else "bg-gray-600"