Also, I feel like I should clarify that when I say tab, I don't actually mean a tab, just a tab shaped button that leads to a new layout.
You'll need at least one record in your found set in order to use this method. Can you describe the circumstances where you are getting an empty found set and why? (perhaps there's an alternative approach that will work for you.)
Yes, I kind of mentioned this in the first post, but I might not have explained it clearly. I have a script that runs on entering the layout that searches for shipments that have not yet shipped out. If all of the shipments have shipped out, then it would pull up 0 records. It's not something that happens all the time, but might happen once a day or so.
But this need not be shown as a found set in list view. You might consider using a portal where the relationship matches only to unshipped shipments. This would avoid the issue with a zero found set. Then you can have both portals on a layout where it always has a found set of at least one record.
Another trick is to add one "dummy record" that is never shipped and thus you never have an empty found set, but making sure that you always have this dummy record yet keep it out of reports and such can introduce its own kind of problems.