Quell three new warnings I introduced with AgentPrefsData, return an empty llsd map instead of an empty llsd block when no AgentPrefs service is available to try and appease Firestorm

Signed-off-by: Diva Canto <diva@metaverseink.com>

OpenSim/Data/MySQL/MySQLAgentPreferencesData.cs

@@ -52,9 +52,9 @@ namespace OpenSim.Data.MySQL
             return ret[0];
         }
 
-        public void Store(AgentPreferencesData data)
+        public override bool Store(AgentPreferencesData data)
         {
-            base.Store(data);
+            return base.Store(data);
         }
     }
 }