Specific use case: I'd like to be able to find pages with the page title MediaWiki:Mainpage-title(:?-loggedin)? and which are the empty string. When this message is the empty string, it removes the h1 from the main page (implemented in T255682). I was trying to provide evidence in T293470 that there were other wikis than en.wp that do this (I've added a link to key text in Common.css [which is the legacy way to do it] that indicates a sizeable chunk of the fleet does that, so that need is not pressing).

General use case: All empty pages? Not sure how useful that is for global search.

Right now, the UI prevents a no-text search. You have to put something in the search box. So I'm pretty sure that global search can't find such pages. I tried to hack around it with a regex search for ^$, but that didn't work.