Revert to use merge requests count for group view

- We should not use the variable @merge_requests
in this condition because it only considers open
merge requests and it is recalculated if filters
are present

Changelog: fixed

app/views/groups/merge_requests.html.haml

@@ -2,7 +2,7 @@
 
 - page_title _("Merge requests")
 
-- if @merge_requests&.size == 0
+- if issuables_count_for_state(:merge_requests, :all) == 0
   = render 'shared/empty_states/merge_requests', project_select_button: true
 - else
   .top-area

spec/features/groups/merge_requests_spec.rb

@@ -75,4 +75,29 @@ RSpec.describe 'Group merge requests page' do
       end
     end
   end
+
+  context 'empty state with no merge requests' do
+    before do
+      MergeRequest.delete_all
+    end
+
+    it 'shows an empty state, button to create merge request and no filters bar', :aggregate_failures, :js do
+      visit path
+
+      expect(page).to have_selector('.empty-state')
+      expect(page).to have_link('Select project to create merge request')
+      expect(page).not_to have_selector('.issues-filters')
+    end
+
+    context 'with no open merge requests' do
+      it 'shows an empty state, button to create merge request and filters bar', :aggregate_failures, :js do
+        create(:merge_request, :closed, source_project: project, target_project: project)
+        visit path
+
+        expect(page).to have_selector('.empty-state')
+        expect(page).to have_link('Select project to create merge request')
+        expect(page).to have_selector('.issues-filters')
+      end
+    end
+  end
 end