Bugfixes for is-not-defined and is-defined searches

Partly solution for #281 (closed).

This may be a naïve solution. It works for me for all the text fields and for tasks without any date fields. However, there may be calendars or servers out there having blank values rather than NULL in the database table (and if so, I also don't know if a blank value should be defined as "not defined" or "defined to be blank"). It does not work when only one of the date fields are given, for reasons described in the bug report.

Includes my pull request for issue #279 (closed), as it's related.