helpers: allow champ without persisted dossier

This fixes the /patron page. I also added an end-to-end test
to avoid breaking this page in the future.

app/helpers/champ_helper.rb

@@ -28,7 +28,7 @@ module ChampHelper
 
   def autosave_available?(champ)
     # FIXME: enable autosave on champs private? once we figured out how to batch audit events
-    champ.dossier.brouillon? && !champ.repetition?
+    champ.dossier&.brouillon? && !champ.repetition?
   end
 
   def geo_area_label(geo_area)

spec/system/patron_spec.rb

@@ -0,0 +1,8 @@
+describe 'Accessing the /patron page:' do
+  scenario 'I can display a page with all form fields and UI elements' do
+    visit patron_path
+    expect(page).to have_text('Icônes')
+    expect(page).to have_text('Formulaires')
+    expect(page).to have_text('Boutons')
+  end
+end