Fix used not set cornercase

wildduck_exporter.py

@@ -58,7 +58,7 @@ async def fetch():
 
     # Merge application specific passwords last used timestamps
     async for a in app.ctx.db.asps.find({"user": {"$in": list(last_login.keys())}}):
-        if a["used"] > last_login[a["user"]]:
+        if a["used"] and a["used"] > last_login[a["user"]]:
             last_login[a["user"]] = a["used"]
 
     for user, dt in last_login.items():