FLINK-11449 has shown parts of the API that have not been designed properly. Especially, direct usages of case classes that will be only in the planner module should not be exposed through the API.
Using null for representing a null literal is currently not supported in the Table API as a general NULL type and/or proper type inference is missing. Therefore, nulls must be typed using Null(Type.STRING). However, this encourages users to use case classes instead of the Scala DSL.
We could implement a implicit object for Null, however, all functions and literals currently start with a lower case except for this object. I would recommend to introduce nullOf(type) instead. This would allow us to have nullOf(type) and null in the future.