In Options > Tags you can enter tags Picard should not overwrite (“Preserve these tags from being cleared or overwritten with MusicBrainz data”). Not quite sure right now whether it will leave blank tags blank, maybe just try it. For a list of tags see http://picard.musicbrainz.org/docs/tags/
I would strongly recommend you to also write the musicbrainz_recordingid and musicbrainz_albumid to the files and not put them on the preserve list. Those tags in the files will enable you to easily retag your files with Picard without having to match them again to the correct album / track.