A --ps cleanup for MDEV-12658 Make the third parameter to LPAD and RPAD optional

"mtr --ps func_str" failed:
pad_str was erroneously appended with a new space on every PS execution.

Adding pad_str.length(0) into fix_length_and_dec() to avoid this.

sql/item_strfunc.cc

@@ -3187,6 +3187,7 @@ void Item_func_pad::fix_length_and_dec()
     if (agg_arg_charsets_for_string_result(collation, &args[0], 1, 1))
       return;
     pad_str.set_charset(collation.collation);
+    pad_str.length(0);
     pad_str.append(" ", 1);
   }