Avoid account properties get cleared

[pdawyndt]

Throughout the semester, the properties (name, email) of some student accounts get cleared. This results in students marked as "n/a" in the user listing of a course:

I guess this may happen when a student tries to logon to Dodona, using an account that has been decomissioned by the institution. This way we loose track about these students, even when the semester/academic year is over (all that remains is a unique ID, but no identity can be linked to that from Dodona).

Proposed solution: Avoid that user account information gets cleared during login, in case no new information is provided.