Investigate use and effects of None path values

[Stiivi]

The HierarchyPath is of type List[str] however there are places where None is used as a path value. Document in the comments below:

• Which methods are producing paths with None values
• Which methods are checking for the None value
• Which methods are using path and might be affected with the None value

After the assessment decide whether to change the HierarchyPath to List[Optional[str]] or keep it as it is.

Some pointers:

There is NULL_PATH_VALUE constant that has value "__null__" which was introduced in bb7bb45fdb15f00fedb672564eb31627ab915cb7.