Add defensive check for null asset in publish

Prevents NoneType subscript error if L2 returns null asset.

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

server.py

@@ -4029,7 +4029,7 @@ async def ui_publish_run(run_id: str, request: Request):
         result = resp.json()
 
         # Pin the output and record L2 share
-        if run.output_hash:
+        if run.output_hash and result.get("asset"):
             await database.update_item_metadata(run.output_hash, ctx.actor_id, pinned=True, pin_reason="published")
             # Record L2 share so UI shows published status
             cache_path = get_cache_path(run.output_hash)