Fix empty labels for `pages:deploy` job

Use description of GenericCommitStatus as label if provided
Fallback to core status labels if not

changelogs/unreleased/40997-gitlab-pages-deploy-jobs-have-a-null-status.yml

+---
+title: Fix empty labels of CI builds for gitlab-pages on pipeline page
+merge_request: 24451
+author:
+type: fixed

lib/gitlab/ci/status/external/common.rb

@@ -6,7 +6,7 @@ module Gitlab
       module External
         module Common
           def label
-            subject.description
+            subject.description || super
           end
 
           def has_details?

spec/lib/gitlab/ci/status/external/common_spec.rb

@@ -11,7 +11,7 @@ describe Gitlab::Ci::Status::External::Common do
   end
 
   subject do
-    Gitlab::Ci::Status::Core
+    Gitlab::Ci::Status::Success
       .new(external_status, user)
       .extend(described_class)
   end
@@ -20,6 +20,14 @@ describe Gitlab::Ci::Status::External::Common do
     it 'returns description' do
       expect(subject.label).to eq external_description
     end
+
+    context 'when description is not set' do
+      let(:external_description) { nil }
+
+      it 'uses core status label' do
+        expect(subject.label).to eq('passed')
+      end
+    end
   end
 
   describe '#has_action?' do