Do not raise when getting value of invalid CI node

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

@@ -55,7 +55,7 @@ module Gitlab
                 end
                 define_method("#{symbol}_value") do
-                  raise Entry::InvalidError unless valid?
+                  return unless valid?
                   @entries[symbol].value if @entries[symbol]
                 end

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

@@ -209,10 +209,8 @@ describe Gitlab::Ci::Config::Node::Global do
     end
     describe '#before_script' do
-      it 'raises error' do
-        expect { global.before_script }.to raise_error(
-          Gitlab::Ci::Config::Node::Entry::InvalidError
-        )
+      it 'returns nil' do
+        expect(global.before_script).to be_nil
       end
     end
   end