Only show up or down votes icon on issue list if there are any

Closes #13649

app/assets/stylesheets/pages/issues.scss

@@ -24,7 +24,7 @@
       display: inline-block;
     }
 
-    .issue-no-comments, .issue-no-votes {
+    .issue-no-comments {
       opacity: 0.5;
     }
   }

app/views/projects/issues/_issue.html.haml

@@ -16,23 +16,15 @@
           = link_to_member(@project, issue.assignee, name: false, title: "Assigned to :name")
 
       - upvotes, downvotes = issue.upvotes, issue.downvotes
-      - if upvotes > 0 || downvotes > 0
+      - if upvotes > 0
         %li
           = icon('thumbs-up')
           = upvotes
-      - else
-        %li{ class: 'issue-no-votes' }
-          = icon('thumbs-up')
-          = upvotes
 
-      - if upvotes > 0 || downvotes > 0
+      - if downvotes > 0
         %li
           = icon('thumbs-down')
           = downvotes
-      - else
-        %li{ class: 'issue-no-votes' }
-          = icon('thumbs-down')
-          = downvotes
 
       - note_count = issue.notes.user.count
       - if note_count > 0