Handle when if project has no repository

lib/api/v3/github.rb

@@ -214,6 +214,8 @@ module API
 
           update_project_feature_usage_for(user_project)
 
+          next [] unless user_project.repo_exists?
+
           branches = ::Kaminari.paginate_array(user_project.repository.branches.sort_by(&:name))
 
           present paginate(branches), with: ::API::Github::Entities::Branch, project: user_project

spec/requests/api/v3/github_spec.rb

@@ -472,6 +472,17 @@ RSpec.describe API::V3::Github do
 
         expect(response).to have_gitlab_http_status(:ok)
       end
+
+      context 'when the project has no repository', :aggregate_failures do
+        let_it_be(:project) { create(:project, creator: user) }
+
+        it 'returns an empty collection response' do
+          jira_get v3_api("/repos/#{project.namespace.path}/#{project.path}/branches", user)
+
+          expect(response).to have_gitlab_http_status(:ok)
+          expect(json_response).to be_empty
+        end
+      end
     end
 
     context 'unauthenticated' do