Fix portrait missing in [message] with empty image= key

This is mostly important for use of the MESSAGE macro.  Use image=none
if no image/portrait is supposed to be shown.

data/lua/wml/message.lua

@@ -14,7 +14,7 @@ end
 local function get_image(cfg, speaker)
 	local image = cfg.image
 
-	if speaker and image == nil then
+	if speaker and (image == nil or image == "") then
 		image = speaker.portrait
 	end