The null is to make a gap. It is basically an empty segment placed on the Evenly position to make a gap on the even distance (Size is controlled by Fixed X size parameter).
Compose create Sequence like Gap+Segment+Gap to make a gap between Corner and Defaults.
Basically, it is always better to leave Segment without random/size transforms and to add a Transform node. That way you can use the same segment for different Generator inputs.