When saving an oar/iar, don't attempt to write out the data byte array if it's empty

On Mono 2.6 (and probably before) this causes a non-obvious IOException

OpenSim/Framework/Serialization/TarArchiveWriter.cs

@@ -208,6 +208,8 @@ namespace OpenSim.Framework.Serialization
                 m_bw.Write(header);
     
                 // Write out data
+                // An IOException occurs if we try to write out an empty array in Mono 2.6
+                if (data.Length > 0)
                     m_bw.Write(data);
     
                 if (data.Length % 512 != 0)