GeoNode#project_ids should return nil when have no restrictions

app/models/geo_node.rb

@@ -109,7 +109,7 @@ class GeoNode < ActiveRecord::Base
   end
 
   def project_ids
-    return [] unless groups.any?
+    return unless groups.any?
 
     groups.flat_map { |group| group.all_projects.pluck(:id) }.uniq
   end

spec/models/geo_node_spec.rb

@@ -319,13 +319,13 @@ describe GeoNode, type: :model do
 
   describe '#project_ids' do
     context 'without group restriction' do
-      it 'returns an empty array' do
-        expect(node.project_ids).to eq([])
+      it 'returns nil' do
+        expect(node.project_ids).to be_nil
       end
     end
 
     context 'with group restrictions' do
-      it 'returns an array with unique project ids' do
+      it 'returns an array with unique project ids that belong to the groups' do
         group_1 = create(:group)
         group_2 = create(:group)
         nested_group_1 = create(:group, parent: group_1)