fix bug

src/client/src/tscSubquery.c

@@ -1941,7 +1941,11 @@ void tscFirstRoundRetrieveCallback(void* param, TAOS_RES* tres, int numOfRows) {
 
           // tag or group by column
           if (TSDB_COL_IS_TAG(pExpr->colInfo.flag) || pExpr->functionId == TSDB_FUNC_PRJ) {
-            memcpy(p + offset, row[i], length[i]);
+            if (row[i] == NULL) {
+              setNull(p + offset, pExpr->resType, pExpr->resBytes);
+            } else {
+              memcpy(p + offset, row[i], length[i]);
+            }
             offset += pExpr->resBytes;
           }
         }