Fix invalid X-Next-Page header for the first page

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/471566

**Problem**

Code responsible for the pagination of the first page always assumes
that the second page exists. As a result, it sets a X-Next-Page even if
there is no second page.

**Solution**

Fix the calculation of `total_pages` and `next_page` values.

Changelog: fixed

lib/gitlab/pagination/gitaly_keyset_pager.rb

@@ -70,10 +70,12 @@ def paginate_first_page_via_gitaly(finder)
         finder.execute(gitaly_pagination: true).tap do |records|
           total = finder.total
           per_page = params[:per_page].presence || Kaminari.config.default_per_page
+          total_pages = (total / per_page.to_f).ceil
+          next_page = total_pages > 1 ? 2 : nil
 
           Gitlab::Pagination::OffsetHeaderBuilder.new(
-            request_context: request_context, per_page: per_page, page: 1, next_page: 2,
-            total: total, total_pages: (total / per_page) + 1
+            request_context: request_context, per_page: per_page, page: 1, next_page: next_page,
+            total: total, total_pages: total_pages
           ).execute
         end
       end

spec/lib/gitlab/pagination/gitaly_keyset_pager_spec.rb

@@ -70,6 +70,11 @@
         context 'when first page is requested' do
           let(:branches) { [branch1, branch2, branch3] }
 
+          before do
+            allow(BranchesFinder).to receive(:===).with(finder).and_return(true)
+            allow(finder).to receive(:total).and_return(branches.size)
+          end
+
           it 'keyset pagination is used with offset headers' do
             allow(request_context).to receive(:request).and_return(fake_request)
             allow(project.repository).to receive(:branch_count).and_return(branches.size)
@@ -85,6 +90,26 @@
 
             pager.paginate(finder)
           end
+
+          context 'when second page does not exist' do
+            let(:base_query) { { per_page: 3 } }
+
+            it 'does not set an invalid X-Next-Page header' do
+              allow(request_context).to receive(:request).and_return(fake_request)
+              allow(project.repository).to receive(:branch_count).and_return(branches.size)
+
+              expect(finder).to receive(:execute).and_return(branches)
+              expect(request_context).to receive(:header).with('X-Per-Page', '3')
+              expect(request_context).to receive(:header).with('X-Page', '1')
+              expect(request_context).to receive(:header).with('X-Next-Page', '')
+              expect(request_context).to receive(:header).with('X-Prev-Page', '')
+              expect(request_context).to receive(:header).with('Link', kind_of(String))
+              expect(request_context).to receive(:header).with('X-Total', '3')
+              expect(request_context).to receive(:header).with('X-Total-Pages', '1')
+
+              pager.paginate(finder)
+            end
+          end
         end
 
         context 'when second page is requested' do