{#translators:||,}{##dramaturgs}
won't show a comma in books with empty fields.
But it looks like you want to use '&' instead of comma when separating groups of people?
I want to learn more about the writing advanced templates in calibre, but have little use for them, so I don't know a lot. I think that to use the list_union class you write a
Template Function and call that in the custom column.