Improve xinerama 02_removed_display error check and message

Author: knixeur

test/integration/xinerama/02_removed_display.lua

@@ -1,7 +1,7 @@
 mod_xinerama.refresh();
 
-if notioncore.find_screen_id(1) then
-  return "New number of screens should be 1, found ", notioncore.find_screen_id(1):name()
+if notioncore.find_screen_id(1) ~= nil then
+  return "New number of screens should be 1, found " .. notioncore.find_screen_id(1):name()
 end
 
 os.execute("cp xinerama/fakexinerama-2monitors xinerama/.fakexinerama");