I’m not sure if this is the “right” approach. If you move content from MW to xwiki you should try adhere as much as possible to the target structure. So if you make the TOC macro more robust to this kind of “wrong-doing” people will not care again, as they did in MW before. Empty bullets are a nice “reminder” that something is just wrong.
Also I never was a friend of the MW convention anyhow because it felt unnatural and pushed the burden to solve a technical problem (getting formatting of heading levels right) on the user. And starting something with 2 feels just unnatural (actually starting things with 1 is also, if you are an IT guy ).