If GetAgents() is called with an empty userIDs array then don't bother with a useless potentially network call on the scene presence service connector.

This also eliminates the "[PRESENCE HANDLER]: GetAgents called without required uuids argument"
which has started to pop up in the logs when a call is made with an empty uuid array as occasionally happens.

OpenSim/Region/CoreModules/ServiceConnectorsOut/Presence/BasePresenceServiceConnector.cs

@@ -125,6 +125,10 @@ namespace OpenSim.Region.CoreModules.ServiceConnectorsOut.Presence
 
         public PresenceInfo[] GetAgents(string[] userIDs)
         {
+            // Don't bother potentially making a useless network call if we not going to ask for any users anyway.
+            if (userIDs.Length == 0)
+                return new PresenceInfo[0];
+
             return m_PresenceService.GetAgents(userIDs);
         }