A fairly standard method is to use a horizontal arrow pointing towards the unchanged figure. If you use colour — red and green for up and down (whichever) — "unchanged" could be grey.

*Addendum:* It is important to show something! A gap could be interpreted as "We can't tell whether this has moved," "We know this is out of date," or an image which hasn't loaded yet. Since you are indicating movement in the figures, you do need to indicate that you have assessed movement in all the figures where you have actually done that.