🚀 go-pugleaf

This will get all such text frames except any that might be inline/anchored to type on a path if the path is that of a text frame.

myPIs = app.documents[0].allPageItems; 
for (var j = myPIs.length - 1; j >= 0; j--) { 
  if (myPIs[j].constructor.name == "TextFrame" && myPIs[j].contents == "") { 
    try {myPIs[j].contentType = ContentType.unassigned} catch(e) {}		

  } 
}

Note: if a text frame is threaded to a non-empty story, then you can't change the content type of that frame without first breaking it out of the story. That's what the try/catch is there for. The script just ignores any such frame. However, if the whole story is empty, then the frames will be converted.

Dave