mysql-test/r/csv.result:
  update result file
mysql-test/r/log_tables.result:
  log_tables test contains alter of a CSV table with
  NULLs which results in warnings. In fact this is how
  the bug noticed. So, now when it is fixed we should
  update result file.
mysql-test/t/csv.test:
  add a test for the bug
storage/csv/ha_tina.cc:
  We should write 0 to the data file in the
  case we meet NULL. CSV does not support
  NULL values internally and we shouldn't
  distinguish them from 0 when writing a row
  (the alternative is to implement full NULL
  support). Otherwise other routines (such as
  Field::check_int() become confused).
  In 5.0 NULLs are stored as zeroes. In 5.1 it
  somehow turned into empty string. Which is wrong.