Two ways to define may-have-a (1..0-1) relationship
You've got 2 conflicting design goals here. One is to design a good database. The other is to play nicely with OR mappers. You might be able to shift to a more consistent approach to mapping partial participation constraints, but you will continue to have design conflicts in the future.

One option would be to create a set of views to worm with your ORM, and so abstract the problem away.