On a normal page, you aren't able to search with an empty string. The search button simply won't function. This is clearly intentional, and this is not the problem.
However, if you are on the search results page of another search, then you are allowed to search with an empty string. For example: on any page, search for an arbitrary string (like "chair"). After this, the search results for your string should appear. Now, delete the entire search string such that it is empty. If you perform the search action, the site will perform the search even though you used an empty string.