Research of null value in relation (belongs to many)

public function lieuAffectation()
    {
        return $this->belongsToMany(LieuAffectation::class, LieuAffectationHitories::class);
    }

Tables\Filters\SelectFilter::make('id')
                    ->relationship('lastLieuAffectation', 'name_fr')
                    ->multiple()
                    ->label("Lieu d'affectaion")
                    ->options(
                            LieuAffectation::all()->pluck('name_fr', 'id')->toArray()
//  LieuAffectation::all()->pluck('name_fr', 'id')->prepand('null','')->toArray() 

                    ),