Jon's answer didn't work for me as I got a compiler error CS0453 C# The type must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method
This worked for me though:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<SomeObject>().HasOptional(m => m.somefield);
base.OnModelCreating(modelBuilder);
}