Cluster is missing static attributes if query cluster returns null response

[andythsu]

If http endpoint returns null or empty response, clusterstat is missing static attributes such as proxyTo. These attributes belong to backend and not on cluster stats. This is due to

trino-gateway/gateway-ha/src/main/java/io/trino/gateway/ha/clustermonitor/ClusterStatsHttpMonitor.java

Line 65 in 230439e

return clusterStats.build();

We can add the backend attributes before returning.