Merge branch '40671-fix-empty-blank-flash-messages' into 'master'

Remove blank/empty flash messages caused by nil

Closes #40671

See merge request gitlab-org/gitlab-ce!15658

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