Preserve also the empty tags

Is it possible to preserve also the empty tags, when updating the tags?

For example, in the list of random songs (non-albums) I have sometime the correct album of the first release of the song, sometime the album is missing…

…I want to preserve the existing album tag, but also preserve the empty album tag, because the wrong album could be added, some VA, or soundtrack - and I want to have the first album release for these songs.

Save something to the tag so that it’s not empty. Something like:

$set(tag_name,$if2(%tag_name%,Missing))