Champ Siret: Fix siret-champ save

When a user enter a siret, it calls siret_controller in ajax. The champ value is stil nil, so champ.present? is false because champ.blank is redefined to focus on value. Thus the champ is not updated.

spec/controllers/champs/siret_controller_spec.rb

@@ -6,7 +6,7 @@ describe Champs::SiretController, type: :controller do
 
   describe '#show' do
     let(:dossier) { create(:dossier, user: user, procedure: procedure) }
-    let(:champ) { create(:champ_siret, dossier: dossier) }
+    let(:champ) { create(:champ_siret, dossier: dossier, value: nil, etablissement: nil) }
     let(:params) do
       {
         champ_id: champ.id,