Add CI config node that is unspecified null entry

lib/gitlab/ci/config/node/null.rb

+module Gitlab
+  module Ci
+    class Config
+      module Node
+        ##
+        # This class represents an undefined and unspecified node.
+        #
+        # Implements the Null Object pattern.
+        #
+        class Null < Entry
+          def initialize(config = nil, **attributes)
+            super
+          end
+
+          def value
+            nil
+          end
+
+          def valid?
+            true
+          end
+
+          def errors
+            []
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config/node/null_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Config::Node::Null do
+  let(:null) { described_class.new }
+
+  describe '#leaf?' do
+    it 'is leaf node' do
+      expect(null).to be_leaf
+    end
+  end
+
+  describe '#valid?' do
+    it 'is always valid' do
+      expect(null).to be_valid
+    end
+  end
+
+  describe '#errors' do
+    it 'is does not contain errors' do
+      expect(null.errors).to be_empty
+    end
+  end
+
+  describe '#value' do
+    it 'returns nil' do
+      expect(null.value).to eq nil
+    end
+  end
+end