helpers: don't crash on unknown flash level

Errors generated by the `invisible-catcha` gem are reported as
`flash[:error]` (which differs from Rail's usual `flash[:alert]`).

Avoid crashing when the flash level passed to this helper is not known.

app/helpers/application_helper.rb

@@ -8,12 +8,15 @@ module ApplicationHelper
   end
 
   def flash_class(level, sticky: false, fixed: false)
-    class_names = case level
+    class_names = []
+
+    case level
     when 'notice'
-      ['alert-success']
+      class_names << 'alert-success'
     when 'alert'
-      ['alert-danger']
+      class_names << 'alert-danger'
     end
+
     if sticky
       class_names << 'sticky'
     end

spec/helpers/application_helper_spec.rb

@@ -18,6 +18,12 @@ describe ApplicationHelper do
     end
   end
 
+  describe "#flash_class" do
+    it { expect(flash_class('notice')).to eq 'alert-success' }
+    it { expect(flash_class('alert', sticky: true, fixed: true)).to eq 'alert-danger sticky alert-fixed' }
+    it { expect(flash_class('unknown-level')).to eq '' }
+  end
+
   describe "#try_format_date" do
     subject { try_format_date(date) }