Bug #39265: fix for the bug 33699 should be reverted

Documented behaviour was broken by the patch for bug 33699
that actually is not a bug.

This fix reverts patch for bug 33699 and reverts the
UPDATE of NOT NULL field with NULL query to old
behavior.

mysql-test/r/auto_increment.result

@@ -231,7 +231,8 @@ a	b
 204	7
 delete from t1 where a=0;
 update t1 set a=NULL where b=6;
-ERROR 23000: Column 'a' cannot be null
+Warnings:
+Warning	1048	Column 'a' cannot be null
 update t1 set a=300 where b=7;
 SET SQL_MODE='';
 insert into t1(a,b)values(NULL,8);
@@ -246,7 +247,7 @@ a	b
 1	1
 200	2
 201	4
-203	6
+0	6
 300	7
 301	8
 400	9
@@ -262,7 +263,6 @@ a	b
 1	1
 200	2
 201	4
-203	6
 300	7
 301	8
 400	9
@@ -273,20 +273,20 @@ a	b
 405	14
 delete from t1 where a=0;
 update t1 set a=NULL where b=13;
-ERROR 23000: Column 'a' cannot be null
+Warnings:
+Warning	1048	Column 'a' cannot be null
 update t1 set a=500 where b=14;
 select * from t1 order by b;
 a	b
 1	1
 200	2
 201	4
-203	6
 300	7
 301	8
 400	9
 401	10
 402	11
-404	13
+0	13
 500	14
 drop table t1;
 create table t1 (a bigint);