Deneb Date as Condition

Hi
in a Gantt Diagram I have a date called "Udførsels Start". I want to have a condition for coloring this field, so in case it's has a null value, it should be transperrent - otherwise blue.
The issue is my test does not work. Color is alway blue. What should the test look like?

"color": {
"condition": {
"test": "datum['Udførelse start'] === null",
"value": "transperrent"
},
"value": "blue"
},