Now test for NULLness the pointers returned from objects created from the
default value. Pushing patch on behalf of cmiller.


mysql-test/r/null.result:
  Add test case
mysql-test/t/null.test:
  Add test case
sql/sql_table.cc:
  No longer blindly dereference pointer of the string representation of the
  values, where "NULL" is NUL. Raise INVALID DEFAULT error messages where
  appropriate.
  
  Note that the -O1 optimization flag made debugging this extremely tricky, with
  misleading results, and that removing it from the Makefile during debugging can
  be invaluable.