Correct a bug (that I introduced, after using Oracle's database software for 
too many years) where the length of the database-sent data is incorrectly 
used to infer NULLness.


client/mysql.cc:
  No longer use the length of the data to infer whether it is NULL or not.
mysql-test/r/mysql.result:
  Add result and version marker, and correct previous result.
mysql-test/t/mysql.test:
  Add test and version marker