In a Cards View, if “Show empty Field values” is selected, an empty Assignees field is not shown on the cards:
Is it a LookUp?
Yes, that one is a Lookup.
I think an empty field will only be shown if it is an editable field.
For something like Assignees (editable) it makes sense to show the blank icon so that a user can click on it to add people.
For lookups (or formulas) there seems little point in showing the blank circle since it cannot be interacted with.