[TS-1278]<fix>: fix isNull parameter with cols merge

src/common/src/tdataformat.c

@@ -624,8 +624,9 @@ static void tdMergeTwoDataCols(SDataCols *target, SDataCols *src1, int *iter1, i
       if ((key1 > key2) || (key1 == key2 && !TKEY_IS_DELETED(tkey2))) {
         for (int i = 0; i < src2->numOfCols; i++) {
           ASSERT(target->cols[i].type == src2->cols[i].type);
-          if (src2->cols[i].len > 0 && !isNull(src2->cols[i].pData, src2->cols[i].type)) {
-            dataColAppendVal(&(target->cols[i]), tdGetColDataOfRow(src2->cols + i, *iter2), target->numOfRows,
+          const void *src2Val = tdGetColDataOfRow(src2->cols + i, *iter2);
+          if (src2->cols[i].len > 0 && !isNull(src2Val, src2->cols[i].type)) {
+            dataColAppendVal(&(target->cols[i]), src2Val, target->numOfRows,
                              target->maxPoints, 0);
           } else if(!forceSetNull && key1 == key2 && src1->cols[i].len > 0) {
             dataColAppendVal(&(target->cols[i]), tdGetColDataOfRow(src1->cols + i, *iter1), target->numOfRows,