If you open the release editor for a release (where the release event fields are currently blank, e.g. http://test.musicbrainz.org/release/2440cf4c-2305-48cf-b733-c9d66168c838 ) and click on them (so that you get a form object which is blank), and submit the edit, it will actually enter an edit to change that field from unset to unset. It should only submit an edit if the field was previously set to something.

e.g. for that release, I managed to create the following edits, all of which give errors when I try to display them:
date: http://test.musicbrainz.org/edit/169
packaging: http://test.musicbrainz.org/edit/170
status: http://test.musicbrainz.org/edit/171
language: http://test.musicbrainz.org/edit/172
script: http://test.musicbrainz.org/edit/173
country: http://test.musicbrainz.org/edit/174
barcode: http://test.musicbrainz.org/edit/175