Fix crash updating legislation process categories

We were expecting translation parameters in legislation processes
`update` action. However, those parameters aren't sent when we get to
that action through the "proposals" tab.

app/controllers/concerns/translatable.rb

@@ -8,6 +8,8 @@ module Translatable
   private
 
     def translation_params(resource_model)
+      return [] unless params[:enabled_translations]
+
       resource_model.translated_attribute_names.product(enabled_translations).map do |attr_name, loc|
         resource_model.localized_attr_name_for(attr_name, loc)
       end

spec/features/admin/legislation/processes_spec.rb

@@ -130,5 +130,16 @@ feature 'Admin legislation processes' do
 
       expect(page).not_to have_content 'Draft publication'
     end
+
+    scenario "Change proposal categories" do
+      visit edit_admin_legislation_process_path(process)
+      within(".admin-content") { click_link "Proposals" }
+
+      fill_in "Categories", with: "recycling,bicycles"
+      click_button "Save changes"
+
+      visit admin_legislation_process_proposals_path(process)
+      expect(page).to have_field("Categories", with: "bicycles, recycling")
+    end
   end
 end