Added ForeignKey allow null value
Added ForeignKey allow null value

hannesk001/SPHERE-Framework,DerDu/SPHERE-Framework,DerDu/SPHERE-Framework,hannesk001/SPHERE-Framework,hannesk001/SPHERE-Framework,BozzaCoon/SPHERE-Framework,DerDu/SPHERE-Framework,DerDu/SPHERE-Framework,DerDu/SPHERE-Framework,BozzaCoon/SPHERE-Framework,DerDu/SPHERE-Framework,BozzaCoon/SPHERE-Framework,BozzaCoon/SPHERE-Framework,hannesk001/SPHERE-Framework,BozzaCoon/SPHERE-Framework,hannesk001/SPHERE-Framework,hannesk001/SPHERE-Framework,BozzaCoon/SPHERE-Framework,BozzaCoon/SPHERE-Framework,DerDu/SPHERE-Framework,hannesk001/SPHERE-Framework