MDEV-9550 COUNT(NULL) returns incorrect result with sequence storage engine

when calculating COUNT(basic_const), take into account that
this basic_const may be NULL

storage/sequence/mysql-test/sequence/group_by.result

@@ -85,4 +85,10 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 explain select count(*) from seq_1_to_15_step_2 group by mod(seq,2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	seq_1_to_15_step_2	index	NULL	PRIMARY	8	NULL	8	Using index; Using temporary; Using filesort
-drop table seq_1_to_15_step_2;
+create temporary table t1 select * from seq_1_to_3;
+select count(NULL) from t1;
+count(NULL)
+0
+select count(NULL) from seq_1_to_3;
+count(NULL)
+0

storage/sequence/mysql-test/sequence/group_by.test

@@ -39,4 +39,9 @@ explain select count(*) from seq_1_to_15_step_2, seq_1_to_15_step_2 as t2;
 explain select count(*) from seq_1_to_15_step_2 where seq > 0;
 explain select count(*) from seq_1_to_15_step_2 group by mod(seq,2);
 
-drop table seq_1_to_15_step_2;
+#
+# MDEV-9550 COUNT(NULL) returns incorrect result with sequence storage engine
+#
+create temporary table t1 select * from seq_1_to_3;
+select count(NULL) from t1;
+select count(NULL) from seq_1_to_3;

storage/sequence/sequence.cc

@@ -452,7 +452,11 @@ int ha_seq_group_by_handler::next_row()
     switch (item_sum->sum_func()) {
     case Item_sum::COUNT_FUNC:
     {
-      field->store((longlong) elements, 1);
+      Item *arg0= ((Item_sum*) item_sum)->get_arg(0);
+      if (arg0->basic_const_item() && arg0->is_null())
+        field->store(0LL, 1);
+      else
+        field->store((longlong) elements, 1);
       break;
     }
     case Item_sum::SUM_FUNC: