Allow NULL Foreign Keys with SQLite-net-extensions

I'd like to know if I can, with a SQLite database and SQLite-Net Exensions, add 2 foreign keys to the same table with, each time, one of the foreign keys empty.

My structure is the following:

[Table("Picture")]
public class Picture
{
    [PrimaryKey]
    public int Id { get; set; }

    public string Name { get; set; }

    [ForeignKey(typeof(Contact))] // => Allow Null ?
    public string TokenContact { get; set; }

    [ForeignKey(typeof(Profile))] // Allow Null ?
    public string TokenProfile { get; set; }
}

[Table("Contact")]
public class Contact
{
    [PrimaryKey]
    public string Token {get;set;}

    [OneToMany]   
    public ObservableCollection<Picture> CollectionPicture {get; set;}
 }

[Table("Profile")]
public class Profile:Contact
{

// Some other properties...

 }

Thanks for your advices !