Hi @peter_brueck,
yes, handling null or empty values in Notion nodes can be a bit tricky. We have identified some work-arounds for this in the past.
Check out the following forum posts and see if they can solve your problem as well 
- (Create Notion database page with multi-select property - error if empty)
- (Notion Empty Value Leading to Error)
- (Null value in Notion Node causing Error)
Do let us know how you get on!