use prepare_table() only with a database and not for flat strings

Model::prepare_table() will create a log: "you should provide the table name...."
and it is not needed if a Model object is used only for flat strings

src/model.cpp

@@ -200,7 +200,7 @@ void Model::to_text(PT::TextStream & stream, ModelData * model_data, bool clear_
 		{
 			try
 			{
-				prepare_table();
+				// prepare_table(); at the moment flat strings (json/space) do not need a table name
 				flat_connector->to_text(stream, *this);
 			}
 			catch(...)