Add a null token to pipelines expressions

lib/gitlab/ci/pipeline/expression/null.rb

+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Null < Expression::Token
+          PATTERN = /null/.freeze
+
+          def initialize(value)
+            @value = value
+          end
+
+          def evaluate(**_)
+            nil
+          end
+        end
+      end
+    end
+  end
+end