Use default_branch for code coverage

This commits allow to fetch our code coverage data for the
default_branch with our graphql endpoint.

ee/app/graphql/ee/types/project_type.rb

@@ -153,6 +153,7 @@ module EE
             results = ::Ci::DailyBuildGroupReportResult
               .by_projects(project_ids)
               .with_coverage
+              .with_default_branch
               .latest
               .summaries_per_project
 

ee/spec/requests/api/graphql/project/code_coverage_summary_spec.rb

@@ -37,14 +37,26 @@ RSpec.describe 'Getting code coverage summary in a project' do
   end
 
   context 'when project has coverage' do
-    let!(:daily_build_group_report_result) { create(:ci_daily_build_group_report_result, project: project) }
+    context 'for the default branch' do
+      let!(:daily_build_group_report_result) { create(:ci_daily_build_group_report_result, project: project) }
 
-    it 'contains code coverage summary data', :aggregates_failures do
-      post_graphql(query, current_user: current_user)
+      it 'contains code coverage summary data', :aggregates_failures do
+        post_graphql(query, current_user: current_user)
+
+        expect(code_coverage_summary_graphql_data.dig('averageCoverage')).to eq(77.0)
+        expect(code_coverage_summary_graphql_data.dig('coverageCount')).to eq(1)
+        expect(code_coverage_summary_graphql_data.dig('lastUpdatedAt')).to eq(daily_build_group_report_result.date.to_s)
+      end
+    end
+
+    context 'not for the default branch' do
+      let!(:daily_build_group_report_result) { create(:ci_daily_build_group_report_result, :on_feature_branch, project: project) }
+
+      it 'returns nil' do
+        post_graphql(query, current_user: current_user)
 
-      expect(code_coverage_summary_graphql_data.dig('averageCoverage')).to eq(77.0)
-      expect(code_coverage_summary_graphql_data.dig('coverageCount')).to eq(1)
-      expect(code_coverage_summary_graphql_data.dig('lastUpdatedAt')).to eq(daily_build_group_report_result.date.to_s)
+        expect(code_coverage_summary_graphql_data).to be_nil
+      end
     end
   end