Not sure I understand your question, so here's my statement of the problem: A Role can be none, one, or both of two 'types'.
If that's so, then yes they should both be nullable. If the role has to have at least one assigned (both cannot be null), then depending on the DBMS you may be able to express that as a constraint on the table.
Hope that helps.
Added: As it's an either/or and not both, but must be one, I don't know how to show that on an ERD. In SQL Server I would use a Check Constraint, like:
ALTER TABLE [config].[Generator] WITH CHECK ADD CONSTRAINT [CK_Generator] CHECK
(( ([PNWriterProviderId] IS NOT NULL OR [BodWriterProviderId] IS NOT NULL)
and
([PNWriterProviderId] IS NULL OR [BodWriterProviderId] IS NULL)))
GO
(An example from a table I have.) Here one must not be null, and one must be null.
How to put that on an ERD diagram, as I say, I don't know.
Cheers -