list.sh: only show headings when there are items to show

If there are no formulae or casks to show, then don't show the
"Formulae" or "Casks" headings. This used to be the case when there was
only the Ruby implementation, and continues to be the case when the Ruby
implementation is used [^1], so we should maintain the consistency.

[^1]: https://github.com/Homebrew/brew/blob/4a95077682ae5f342cf60ff181ebbcfb03d49117/Library/Homebrew/cmd/list.rb#L162-L170