MDEV-31277 Wrong result on 2-nd execution of PS to select from view using derived

As a result of this bug the second execution of the prepared statement
created for select from materialized view could return a wrong result set if
- the specification of the view used a left join
- an inner table the left join was a mergeable derived table
- the derived table contained a constant column.

The problem appeared because the flag 'maybe-null' of the wrapper
Item_direct_view_ref constructed for the constant field of the mergeable
derived table was not set to 'true' on the second execution of the
prepared statement.

The patch always sets this flag properly when calling the function
Item_direct_view_ref::set_null_ref-table(). The latter is invoked in
Item_direct_view_ref constructor if it is created for some reference of
a constant column belonging to a mergeable derived table.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

mysql-test/main/join_outer.test

@@ -2372,11 +2372,8 @@ create view v1 as select * from t1
   left join ( select 'Y' AS Voted, ElectionID from t2 ) AS T 
     on T.ElectionID = t1.Election
 limit 9;
-#enable after fix MDEV-31277
---disable_ps2_protocol
 # limit X causes merge algorithm select as opposed to temp table
 select * from v1;
---enable_ps2_protocol
 drop table t1, t2;
 drop view v1;
 
@@ -2391,10 +2388,7 @@ create view v10 as select *, 'U' as u from t10 left join (select 'Y' as y, t20.b
 create table t30 (c int);
 insert into t30 values (1),(3);
 create view v20 as select * from t30 left join (select 'X' as x, v10.u, v10.y, v10.b from v10) dt2 on t30.c=dt2.b limit 6;
-#check after fix MDEV-31277
---disable_ps2_protocol
 select * from v20 limit 9;
---enable_ps2_protocol
 drop view v10, v20;
 drop table t10, t20, t30;
 
@@ -2408,8 +2402,6 @@ insert into t3 values (3),(1);
 create table t1 (a int);
 insert into t1 values (1),(2),(7),(1);
 
-#check after fix MDEV-31277
---disable_ps2_protocol
 select * from
 (
   select * from
@@ -2422,7 +2414,6 @@ select * from
     on dt1.a=dt2.b
   limit 9
 ) dt;
---enable_ps2_protocol
 
 ## Same as dt3 above
 create view v3(x,c) as select * from (select 'X' as x, t3.c from t3) dt3;
@@ -2436,10 +2427,7 @@ create view v0(y,b,x,c) as select * from v2 left join v3 on v2.b=v3.c;
 # Same as above select statement
 create view v1 as  select 'Z' as z, t1.a, v0.*  from t1 left join v0 on t1.a=v0.b limit 9;
 
-#check after fix MDEV-31277
---disable_ps2_protocol
 select * from v1;
---enable_ps2_protocol
 
 set statement join_cache_level=0 for
 select * from v1;