Add a spec for #20079.

The issue was fixed in 2ba5e625. The spec is going in separately just
so the fix could go in as soon as possible.

features/project/wiki.feature

@@ -8,6 +8,12 @@ Feature: Project Wiki
     Given I create the Wiki Home page
     Then I should see the newly created wiki page
 
+  Scenario: Add new page with errors
+    Given I create the Wiki Home page with no content
+    Then I should see a "Content can't be blank" error message
+    When I create the Wiki Home page
+    Then I should see the newly created wiki page
+
   Scenario: Pressing Cancel while editing a brand new Wiki
     Given I click on the Cancel button
     Then I should be redirected back to the Edit Home Wiki page

features/steps/project/wiki.rb

@@ -19,6 +19,11 @@ class Spinach::Features::ProjectWiki < Spinach::FeatureSteps
     click_on "Create page"
   end
 
+  step 'I create the Wiki Home page with no content' do
+    fill_in "wiki_content", with: ''
+    click_on "Create page"
+  end
+
   step 'I should see the newly created wiki page' do
     expect(page).to have_content "Home"
     expect(page).to have_content "link test"
@@ -173,6 +178,11 @@ class Spinach::Features::ProjectWiki < Spinach::FeatureSteps
     find('a[href*="?version_id"]')
   end
 
+  step 'I should see a "Content can\'t be blank" error message' do
+    expect(page).to have_content('The form contains the following error:')
+    expect(page).to have_content('Content can\'t be blank')
+  end
+
   def wiki
     @project_wiki = ProjectWiki.new(project, current_user)
   end