Fix 500 error updating mirror URLs for projects

CHANGELOG-EE

 Please view this file on the master branch, on stable branches it's out of date.
 v 8.13.0 (unreleased)
+  - Fix 500 error updating mirror URLs for projects
 v 8.12.4 (unreleased)
   - [ES] Indexer works with smaller batches of repositories to not exceed NOFILE limit

app/models/project.rb

@@ -527,7 +527,7 @@ class Project < ActiveRecord::Base
   end
   def import_url
-    if import_data && super
+    if import_data && super.present?
       import_url = Gitlab::UrlSanitizer.new(super, credentials: import_data.credentials)
       import_url.full_url
     else

spec/requests/projects/mirrors_controller_spec.rb

+require 'spec_helper'
+
+describe Projects::MirrorsController do
+  let(:project) do
+    create(:project,
+           mirror: true,
+           mirror_user: user,
+           import_url: 'http://user:pass@test.url')
+  end
+  let(:user) { create(:user) }
+
+  describe 'updates the mirror URL' do
+    before do
+      project.team << [user, :master]
+      login_as(user)
+    end
+
+    it 'complains about passing an empty URL' do
+      patch namespace_project_mirror_path(project.namespace, project),
+        project: {
+        mirror: '1',
+        import_url: '',
+        mirror_user_id: '1',
+        mirror_trigger_builds: '0'
+      }
+
+      expect(response).to have_http_status :success
+      expect(response).to render_template(:show)
+      expect(response.body).to include('Import url can&#39;t be blank')
+    end
+  end
+end