fix(linked_dropdown/dropdown): for dropdown_list, drop blank option because rails add it when the field is required. kind of same behaviour for linked_dropdown_list [but requires to manage it on the front too]

spec/system/users/linked_dropdown_spec.rb

@@ -64,6 +64,7 @@ describe 'linked dropdown lists' do
       expect(page).to have_select("Valeur secondaire dépendant de la première", options: ['', 'Secondary 1.1', 'Secondary 1.2'])
     end
   end
+
   private
 
   def log_in(email, password, procedure)