Use nil? for ref evaluation

app/views/projects/pipeline_schedules/_pipeline_schedule.html.haml

@@ -4,7 +4,7 @@
       = pipeline_schedule.description
     %td.branch-name-cell
       = icon('code-fork')
-      - if pipeline_schedule.ref.present?
+      - unless pipeline_schedule.ref.nil?
         = link_to pipeline_schedule.ref, project_ref_path(@project, pipeline_schedule.ref), class: "ref-name"
     %td
       - if pipeline_schedule.last_pipeline