Make sure IF_NULL_ROW expressions receive a separate slot in the sorter used

to implement GROUP BY.

FossilOrigin-Name: 2bda4fca06ab6be5ad02377a7d1fd9fb9586e3181f1052e4b4937958bdd45efe

src/select.c

@@ -3798,6 +3798,7 @@ static Expr *substExpr(
           ifNullRow.op = TK_IF_NULL_ROW;
           ifNullRow.pLeft = pCopy;
           ifNullRow.iTable = pSubst->iNewTable;
+          ifNullRow.iColumn = -99;
           ifNullRow.flags = EP_IfNullRow;
           pCopy = &ifNullRow;
         }