Fix link to blank group icon

When the group is the default blank icon, this needs to use the
`image_path` helper; otherwise, the link won't work if assets are
precompiled. This still works fine for uploaded icons in either case.

CHANGELOG

@@ -19,6 +19,7 @@ v 8.9.0 (unreleased)
   - Fix issues filter when ordering by milestone
   - Todos will display target state if issuable target is 'Closed' or 'Merged'
   - Fix bug when sorting issues by milestone due date and filtering by two or more labels
+  - Link to blank group icon doesn't throw a 404 anymore
   - Remove 'main language' feature
   - Pipelines can be canceled only when there are running builds
   - Use downcased path to container repository as this is expected path by Docker

app/helpers/groups_helper.rb

@@ -31,7 +31,7 @@ module GroupsHelper
     if group && group.avatar.present?
       group.avatar.url
     else
-      'no_group_avatar.png'
+      image_path('no_group_avatar.png')
     end
   end