fixbug: there is a ',' before 'quantile=' when labels is empty (#2659)

Co-authored-by: Renzhong Zhang <renzhong915@gmail.com>

src/bvar/multi_dimension_inl.h

@@ -372,7 +372,7 @@ void MultiDimension<T>::make_labels_kvpair_string(std::ostream& os,
         comma[0] = ',';
     }
     if (quantile > 0) {
-        os << ",quantile=\"" << quantile << "\"";
+        os << comma << "quantile=\"" << quantile << "\"";
     }
     os << "}";
 }