[4.2] Allow remember token to be stored in the database with a non-null default

Checking is_null here would make it so the user would not be remembered after the first login if you have the remember_token field default set to anything but null in the database.

Storing non-null values as default in the database is useful for high-performance applications, see:

http://stackoverflow.com/questions/1106258/mysql-null-vs/12706491#12706491

which cites:

http://shop.oreilly.com/product/0636920022343.do (High Performance MySQL Book)