is there any reason to prefer null over empty collection as return value in kotlin?

I think it is not specific for any language, it is related for business logic

There are situations where both

null

and "empty collection" can be useful values, e.g.

null

meaning "not initialized yet" and "empty collection" meaning "initialized, but empty".

yes, null for collections as "not initialized" can be (and should be) for domain object properties, but for business logic results (data, returned from services, etc) should be empty collection