Use shortcut for string join

Co-authored-by: Frédéric Delaporte <12201973+fredericDelaporte@users.noreply.github.com>

Author: hazzik

src/NHibernate/Dialect/DB2Dialect.cs

@@ -251,7 +251,7 @@ public override SqlString GetLimitString(SqlString sql, SqlString offset, SqlStr
 			SqlStringBuilder pagingBuilder = new SqlStringBuilder();
 			pagingBuilder
 				.Add("select " )
-				.Add(string.Join(",", selectColumns))
+				.Add(selectColumns)
 				.Add(" from (select ")
 				.Add(rownumClause)
 				.Add(sql.Substring(7))