bugfix: join a=b where cast(a as type_of_b) can produce NULL

optimizer implicitly assumed that if `a` in `a=b` is not NULL,
then it's safe to convert `a` to the type of `b` and search the
result in the index(b).

which is not always the case, as converting a non-null value
to a different type might produce NULL. And searching for NULL
in the index might find NULL there, so NULL will be equal to NULL,
making `a=b` behave as if it was `a<=>b`

mysql-test/main/subselect_cache.result

@@ -1698,7 +1698,7 @@ Subquery_cache_miss	18
 show status like '%Handler_read%';
 Variable_name	Value
 Handler_read_first	0
-Handler_read_key	32
+Handler_read_key	29
 Handler_read_last	0
 Handler_read_next	0
 Handler_read_prev	0
@@ -1763,7 +1763,7 @@ Subquery_cache_miss	10
 show status like '%Handler_read%';
 Variable_name	Value
 Handler_read_first	0
-Handler_read_key	13
+Handler_read_key	12
 Handler_read_last	0
 Handler_read_next	0
 Handler_read_prev	0