WHERE column_name = key_column_name was calculated as true
for NULL values.


Docs/manual.texi:
  Changelog
mysql-test/r/distinct.result:
  Updated results caused by bug fix.
mysql-test/r/null_key.result:
  New tests
mysql-test/t/null_key.test:
  New tests
sql/sql_select.cc:
  Additional change for previous changeset for using BLOB in GROUP BY