fix: remove needless empty line from serialization

toml/serializer.hpp

@@ -411,8 +411,11 @@ struct serializer
             token += key_and_sep;
             token += visit(serializer(residual_width, this->float_prec_, true),
                            kv.second);
+            if(token.back() != '\n')
+            {
                 token += '\n';
             }
+        }
 
         // normal tables / array of tables