the result takes its charset/collation
attributes from the character string,
e.g.  SELECT func(NULL, _latin2'string')
now returns a latin2 result. This is
done by introducing a new derivation
(aka coercibility) level DERIVATION_IGNORABLE,
which is used with Item_null.
2. 'Pure' NULL is now BINARY(0), not CHAR(0).
I.e. NULL is now more typeless.


mysql-test/r/metadata.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/r/null.result:
  Testing mixing NULL with a character string with a number of functions.
mysql-test/r/ps_2myisam.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/r/ps_3innodb.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/r/ps_4heap.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/r/ps_5merge.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/r/ps_6bdb.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/r/ps_7ndb.result:
  Fixing test results:
  CHAR(0) -> BINARY(0) for NULLs
mysql-test/t/null.test:
  Testing mixing NULL with a character string with a number of functions.
sql/item.cc:
  New derivation level.
sql/item.h:
  New derivation level.