Allow setting column to NULL.

Special handling of NULL to make it possible to set columns to NULL.

This changes the behaviour of BO slightly, e.g:

* a text column set to null in database would remain null if set to '' in BO, now it will be set to an empty string.
* a int column set to null in BO would be set to 0, now it would be set to null.
Merge pull request #22 from ext/master

Allow setting column to NULL