Show a dash '-' with an empty association

Closes #2681

lib/rails_admin/config/fields/association.rb

@@ -18,7 +18,7 @@ def association
             wording = associated.send(amc.object_label_method)
             can_see = !am.embedded? && (show_action = v.action(:show, am, associated))
             can_see ? v.link_to(wording, v.url_for(action: show_action.action_name, model_name: am.to_param, id: associated.id), class: 'pjax') : ERB::Util.html_escape(wording)
-          end.to_sentence.html_safe
+          end.to_sentence.html_safe.presence || '-'
         end
 
         # Accessor whether association is visible or not. By default

spec/rails_admin/config/fields/association_spec.rb

@@ -4,15 +4,26 @@
   describe '#pretty_value' do
     let(:player) { FactoryBot.create(:player, name: '<br />', team: FactoryBot.create(:team)) }
     let(:field) { RailsAdmin.config('Team').fields.detect { |f| f.name == :players } }
-    let(:view) { ActionView::Base.empty.tap { |d| allow(d).to receive(:action).and_return(nil) } }
+    let(:view) { ActionView::Base.empty }
     subject { field.with(object: player.team, view: view).pretty_value }
 
     context 'when the link is disabled' do
+      let(:view) { ActionView::Base.empty.tap { |d| allow(d).to receive(:action).and_return(nil) } }
+
       it 'does not expose non-HTML-escaped string' do
         is_expected.to be_html_safe
         is_expected.to eq '&lt;br /&gt;'
       end
     end
+
+    context 'when the value is empty' do
+      let(:team) { FactoryBot.build :team }
+      subject { field.with(object: team, view: view).pretty_value }
+
+      it "returns '-' to show emptiness" do
+        is_expected.to eq '-'
+      end
+    end
   end
 
   describe '#removable?', active_record: true do