france_connect: fix for params entirely missing from the callback

Fix a Sentry exception encountered in production.

app/controllers/france_connect/particulier_controller.rb

@@ -30,7 +30,7 @@ class FranceConnect::ParticulierController < ApplicationController
   private
 
   def redirect_to_login_if_fc_aborted
-    if params[:code].empty?
+    if params[:code].blank?
       redirect_to new_user_session_path
     end
   end

spec/controllers/france_connect/particulier_controller_spec.rb

@@ -25,6 +25,12 @@ describe FranceConnect::ParticulierController, type: :controller do
 
     subject { get :callback, params: { code: code } }
 
+    context 'when params are missing' do
+      subject { get :callback }
+
+      it { is_expected.to redirect_to(new_user_session_path) }
+    end
+
     context 'when param code is missing' do
       let(:code) { nil }