Its is impossible to switch projects when a manual contributor was added

[ValentinFutterer]

Describe the bug
If you try to switch projects, you get an error when a manual contributor is assigned to the project. This happens, because the updateProjectLead method is called, when the project is switched. It tries to use the universityId of the contributor, but its null since he/she was added manually. See line 300 in DMPService. Add a null check, to prevent this from happening

Is this a regression?
No.

Steps to Reproduce
Add a manual project.
Add a manual contributor.
Delete the project, save.
Add another project and try to save.

Expected behavior
Switching projects should be no issue.

Environment

• Damap version 4.3.1