Fix NPE in getSupervisorPageInfo for unknown hostnames

[jnioche]

Summary

Fix a NullPointerException in Nimbus.getSupervisorPageInfo() when querying by a hostname with no supervisor registered in the cluster.

Bug

getSupervisorPageInfo is called by the Storm UI's /api/v1/supervisor endpoint. When superId is null (query by hostname), hostToSuperId.get(host) returns null for unknown hosts. The subsequent for-each loop dereferences it:

supervisorIds = hostToSuperId.get(host);  // null if host not in cluster
// ...
for (String sid : supervisorIds) {        // NPE

This is easily triggered by bookmarked URLs for decommissioned supervisors or hostname typos.

Fix

Guard against null with Collections.emptyList(), returning an empty SupervisorPageInfo instead of crashing

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