omnisearch: string fkey to table with no label column is omitted

[cldellow]

CREATE TABLE other(
  slug text not null primary key,
  a text,
  b text
);

CREATE TABLE focus(
  other_slug text not null references other(slug)
);

omnisearch won't autosuggest anything for other_slug due to no label column on other. Since it's a string, we may as well autosuggest the string.