character set with NULL, @A should be latin2
after this query sequence:

   SET @A=_latin2'string';
   SET @A=NULL;

I.e. the second query should not change the charset
to the current default value, but should keep the
original value assigned during the first query.
In order to do it, we don't copy charset
from the argument if the argument is NULL
and the variable has previously been initialized.


mysql-test/r/ps_6bdb.result:
  t
mysql-test/r/ps_5merge.result:
  t
mysql-test/r/ps_3innodb.result:
  t
mysql-test/r/ps_7ndb.result:
  t
mysql-test/r/ps_4heap.result:
  t
mysql-test/r/ps_2myisam.result:
  t
sql/item_func.cc:
  t