There is an issue when using the visual editor that the state value is not being saved and being left with a null:
condition: not
conditions:
- condition: state
entity_id: calendar.holidays_in_united_kingdom
attribute: description
state: ""
Using the Edit in YAML option solves the issue:
condition: not
conditions:
- condition: state
entity_id: calendar.holidays_in_united_kingdom
attribute: description
state: Public holiday