MDEV-26786 Inserting NULL into base column breaks NATURAL_SORT_KEY column

When returning non-null value from natural_sort_key, make sure Item::null_value is false.

Author: vaintroub

mysql-test/main/natural_sort_key.result

@@ -190,3 +190,16 @@ pic120	pic2120
 pic121	pic2121
 pic02000	pic32000
 tom	tom
+create table t (a varchar(8), b varchar(8) as (natural_sort_key(a)));
+insert into t (a) values ('a2'),(NULL),('a11');
+select * from t order by b;
+a	b
+NULL	NULL
+a2	a02
+a11	a111
+select a, b from t order by b;
+a	b
+NULL	NULL
+a2	a02
+a11	a111
+drop table t;

mysql-test/main/natural_sort_key.test

@@ -86,3 +86,10 @@ SELECT RPAD(val,20,' ') value, NATURAL_SORT_KEY(val) FROM
 ('a01b3')
 )AS data ORDER BY 2,1;
 
+# MDEV-27686 (null value indicator not always reset)
+create table t (a varchar(8), b varchar(8) as (natural_sort_key(a)));
+insert into t (a) values ('a2'),(NULL),('a11');
+select * from t order by b;
+select a, b from t order by b;
+drop table t;
+

sql/item_strfunc.cc

@@ -5682,7 +5682,7 @@ String *Item_func_natural_sort_key::val_str(String *out)
  if (tmp.copy(out, cs, &errs) || out->copy(tmp))
  goto error_exit;
  }
-
+ null_value= false;
  return out;
 
 error_exit: