Fix cache_collection crashing when given an empty collection (#15921)

* Fix cache_collection crashing when given an empty collection

* Add tests

spec/controllers/concerns/cache_concern_spec.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe CacheConcern, type: :controller do
+  controller(ApplicationController) do
+    include CacheConcern
+
+    def empty_array
+      render plain: cache_collection([], Status).size
+    end
+
+    def empty_relation
+      render plain: cache_collection(Status.none, Status).size
+    end
+  end
+
+  before do
+    routes.draw do
+      get  'empty_array' => 'anonymous#empty_array'
+      post 'empty_relation' => 'anonymous#empty_relation'
+    end
+  end
+
+  describe '#cache_collection' do
+    context 'given an empty array' do
+      it 'returns an empty array' do
+        get :empty_array
+        expect(response.body).to eq '0'
+      end
+    end
+
+    context 'given an empty relation' do
+      it 'returns an empty array' do
+        get :empty_relation
+        expect(response.body).to eq '0'
+      end
+    end
+  end
+end