1. Place nursery tiles.
2. Deconstruct one of the nursery tiles.
3. Hover over the nursery info icon of that nursery. (2/3 worker ants in this case)
4. Look to the minimap. It should now be displaying a white tile where there is an empty tile.
Thanks for the report on this. Have repeated it myself for confirmation. I'll pass it on - I know the minimap has undergone some reworking recently so perhaps this is fixed already; we'll see with the next update.