Fix CI/CD trigger policy default value

lib/gitlab/ci/config/entry/policy.rb

@@ -44,6 +44,9 @@ module Gitlab
               ['policy has to be either an array of conditions or a hash']
             end
           end
+
+          def self.default
+          end
         end
       end
     end

spec/lib/gitlab/ci/config/entry/policy_spec.rb

@@ -84,4 +84,10 @@ describe Gitlab::Ci::Config::Entry::Policy do
         .to include 'policy has to be either an array of conditions or a hash'
     end
   end
+
+  describe '.default' do
+    it 'does not have a default value' do
+      expect(described_class.default).to be_nil
+    end
+  end
 end