The column's NOT NULL flag doesn't affect what we should print.  Remove the
wrong logic that does check it.

Also, verify that this and the previous two tests print the same data as 
other output formats.


client/mysql.cc:
  Don't check the column's NOT NULL flag when determining whether to print 
  "NULL" or not.
mysql-test/r/mysql.result:
  Add results and confirm that the other output forms give the same results
  for other similar tests.
mysql-test/t/mysql.test:
  Add regression tests for this bug and two similar bugs.