Remove blank flash messages caused by nil

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40671

See
https://gitlab.com/gitlab-org/gitlab-ce/blob/f7254a4060b30e3134c6cf932eaba0fc8e249e9a/app/controllers/sessions_controller.rb#L42
for an example of where we set `flash[:notice] = nil`

app/views/layouts/_flash.html.haml

 .flash-container.flash-container-page
   -# We currently only support `alert`, `notice`, `success`
   - flash.each do |key, value|
-    %div{ class: "flash-#{key}" }
-      %div{ class: (container_class) }
-        %span= value
+    -# Don't show a flash message if the message is nil
+    - if value
+      %div{ class: "flash-#{key}" }
+        %div{ class: (container_class) }
+          %span= value

spec/features/logout_spec.rb

+require 'spec_helper'
+
+describe 'Logout/Sign out', :js do
+  let(:user) { create(:user) }
+
+  before do
+    sign_in(user)
+    visit root_path
+  end
+
+  it 'sign out redirects to sign in page' do
+    gitlab_sign_out
+
+    expect(current_path).to eq new_user_session_path
+  end
+
+  it 'sign out does not show signed out flash notice' do
+    gitlab_sign_out
+
+    expect(page).not_to have_selector('.flash-notice')
+  end
+end