When I type “Middlesex” into the search bar on the website (tested on multiple browsers and devices), and click “about” next to “Middlesex, ON, CA (county)”, it brings up the following page: https://www.inaturalist.org/places/search?q=null
I’m sure it’s possible for staff to query the database for null slugs, but I don’t know if they have the time/inclination to do it. In theory, anyone could do it via the API, but with about 75000 places and almost 100000 projects, it’s probably not a good use of the API.
I don’t have any idea how many of the broken places have admin levels, but curators can’t fix those, only staff.