How to enforce a nullable foreign key?
Yes, why not? Did you try it?

CREATE TABLE dbo.foo(fooid INT PRIMARY KEY);

CREATE TABLE dbo.bar(barid INT PRIMARY KEY,
  fooid INT NULL FOREIGN KEY REFERENCES dbo.foo(fooid));

INSERT dbo.foo SELECT 1;
INSERT dbo.bar SELECT 1,1;
INSERT dbo.bar SELECT 2,NULL; -- succeeds
GO
INSERT dbo.bar SELECT 3,2; -- fails


If this is not what you meant, then please elaborate or re-word your question.