A childless (statement) at-rule that is the last non-comment child but is
followed by comment siblings was stringified without its terminating
semicolon. On re-parse the trailing comments were folded into the
at-rule prelude and dropped, so building such a tree with append() or
insertBefore() silently lost the comment nodes.
Emit the semicolon when a childless at-rule still has following siblings
so the output round-trips.