Add the type method to Entry::Undefined

lib/gitlab/config/entry/undefined.rb

@@ -31,6 +31,10 @@ module Gitlab
           false
         end
 
+        def type
+          nil
+        end
+
         def inspect
           "#<#{self.class.name}>"
         end

spec/lib/gitlab/config/entry/undefined_spec.rb

@@ -40,4 +40,10 @@ RSpec.describe Gitlab::Config::Entry::Undefined do
       expect(entry.specified?).to eq false
     end
   end
+
+  describe '#type' do
+    it 'returns nil' do
+      expect(entry.type).to eq nil
+    end
+  end
 end