models: explicitly mark optional belongs_to relationship as so

This prepares making the `belongs_to` relationship required by default.

app/models/user.rb

@@ -45,8 +45,8 @@ class User < ApplicationRecord
   has_many :dossiers_invites, through: :invites, source: :dossier
   has_many :feedbacks, dependent: :destroy
   has_one :france_connect_information, dependent: :destroy
-  belongs_to :instructeur
-  belongs_to :administrateur
+  belongs_to :instructeur, optional: true
+  belongs_to :administrateur, optional: true
 
   accepts_nested_attributes_for :france_connect_information