Just like production buildings and towers.
Worker icon is showing above guard towers, but missing above empty farms
This will help with the following scenario:
At the start of winter you remove the workers from farms so they still help out with jobs in town. At spring you check your farms for the planned crops, but you forget to put the workers back. Then at the beginning of next winter you get the completely useless warning that there isn’t enough in feed stores. Because at that point it’s too late, you cannot produce them anymore. If there is an icon to indicate no workers you’ll probably notice it during spring, assing the workers back, so animal feed is produced.