Ensure an empty supplement results in glossy returning empty content,
instead of an empty string. Done by returning #none if no article is
wanted.
When the value #none is returned for the article, this can neatly join
with the other values without a trace. In particular, when `term` is
empty content, this ensures that only #metadata remains, exactly like
in the 'hidden' case.
Effectively, now @term[] is identical to @term:hidden, because no empty
string is inserted. This can be seen in the updated test case, where the
spaces around the blank term are now collapsed to a single space by
Typst, instead of keeping two spaces like before.
A next commit will remove :hidden, since it's not orthogonal w.r.t.
other modifiers, while using a supplement is.
swaits
392e8a8122