I don't think this is correct. If you want to make a special world around tar files, then do, but use valid characters. Remember that this string will be passed straight to the server and the client is not allowed to interpret it at all. They are different.
Some servers return the same thing for the two different URLs.
I am not sure which browsers actually support an empty search string. I agreee it is logical to do so. You should NOT assume any convenmtion for what it means. For example, if the server ANDs its terms, you might expect he catalogue, while if it ORs them you would expect nothing back. If you want to make the catlogue available, it is MUCH better to make a link to it from the index cover page.