An Empty slot has no skill in it, and is supposed to be that way. Many builds that are predetermined by the game, such as Norn Blessings, have empty slots in them.
Optional slots are usually used in Player-created builds to denote a skill slot where any of a variety of skills could be placed with equal effectiveness.
You can use these templates to display skills and icons. Some do not work with monster skills such as Knock. The template pages available through the links in the right column often have more thorough documentation that may indicate limitations and additional options. See Category:Templates/Skill display.