Use strings for @scope in ClustersController#index

app/controllers/projects/clusters_controller.rb

@@ -7,7 +7,7 @@ class Projects::ClustersController < Projects::ApplicationController
   before_action :authorize_admin_cluster!, only: [:destroy]
 
   def index
-    @scope = params[:scope] || :all
+    @scope = params[:scope] || 'all'
     clusters = ClustersFinder.new(project, current_user, @scope).execute
     @clusters = clusters.page(params[:page]).per(20)
     @active_count = project.clusters.enabled.count

app/views/projects/clusters/index.html.haml

@@ -16,7 +16,7 @@
       - @clusters.each do |cluster|
         = render "cluster", cluster: cluster
     = paginate @clusters, theme: "gitlab"
-  - elsif @scope == :all
+  - elsif @scope == 'all'
     = render "empty_state"
   - else
     = render "tabs"