Add logic to search for contact details of main unit when there are none on a sub unit

[SandGrainOne]

Description

This issue include some analysis of current functionality in Altinn 2 and possible KoFuVi. Should Register return the contact details of the main unit of a sub unit without contact details? If so then implement necessary changes.

Tasks

• [x] Find out if this issue is something we need to do or not. Investigate whether this is existing logic in Altinn 2 and KoFuVi.
  • This is existing logic in 2.0
• [x] Update service logic in Register to search for contact information of main unit if current organisation is a sub unit without contact information
• [x] QA
• [ ] Documentation
• [x] Manual testing

Acceptance criteria

• [x] Register will return the contact information of the main unit of a sub unit without contact information

[acn-sbuad]

This was implemented as a part of #442 and is handled in the altinn 2 register logic. Info for main unit will be returned if non exist for the sub unit.

[SandGrainOne]

Tested ok