Add Not Null and is Null to Views2 integration

As far as I know this is the last patch I'm going to add to views2 before 3.0 goes live

This patch adds the ability to filter on null and not null (or empty in Views2 terminology) values to most of the fields based on the numeric, string and in handlers (ie everything but boolean and datetime fields), which is most of the field exposed in views2.

2 things missing of note, that I know of
1.) Support for custom fields
2.) As views2 doesn't support it support for datetime fields