MDEV-10236 Where expression with NOT function gives incorrect result

Item_cache::is_null() erroneously returned null_value in a
non-cached state. Now Item_cache::is_null() uses has_value(),
which caches the value if not cached yet, similar to what other value methods do
(e.g. val_int, val_real, etc).

mysql-test/r/null.result

@@ -1572,5 +1572,22 @@ SELECT * FROM t1 WHERE NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(N
 i
 DROP TABLE t1;
 #
+# MDEV-10236 Where expression with NOT function gives incorrect result
+#
+CREATE TABLE t1 (c1 INT);
+INSERT INTO t1 VALUES (1),(2),(3);
+EXPLAIN EXTENDED
+SELECT * FROM t1 WHERE ((c1 IS NOT NULL) >= (NOT TRUE)) IS NOT NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`c1` AS `c1` from `test`.`t1` where (((`test`.`t1`.`c1` is not null) >= <cache>((not(1)))) is not null)
+SELECT * FROM t1 WHERE ((c1 IS NOT NULL) >= (NOT TRUE)) IS NOT NULL;
+c1
+1
+2
+3
+DROP TABLE t1;
+#
 # End of 10.1 tests
 #

mysql-test/t/null.test

@@ -1007,6 +1007,17 @@ SELECT * FROM t1 WHERE NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(NULLIF(N
 DROP TABLE t1;
 
 
+--echo #
+--echo # MDEV-10236 Where expression with NOT function gives incorrect result
+--echo #
+CREATE TABLE t1 (c1 INT);
+INSERT INTO t1 VALUES (1),(2),(3);
+EXPLAIN EXTENDED
+SELECT * FROM t1 WHERE ((c1 IS NOT NULL) >= (NOT TRUE)) IS NOT NULL;
+SELECT * FROM t1 WHERE ((c1 IS NOT NULL) >= (NOT TRUE)) IS NOT NULL;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/item.h

@@ -5050,7 +5050,7 @@ public:
   bool basic_const_item() const
   { return MY_TEST(example && example->basic_const_item()); }
   virtual void clear() { null_value= TRUE; value_cached= FALSE; }
-  bool is_null() { return null_value; }
+  bool is_null() { return !has_value(); }
   virtual bool is_expensive()
   {
     if (value_cached)