Do not raise when adding undefined variables resource

lib/gitlab/ci/variables/collection.rb

@@ -17,6 +17,8 @@ module Gitlab
         end
 
         def concat(resources)
+          return self if resources.nil?
+
           tap { resources.each { |variable| self.append(variable) } }
         end
 

spec/lib/gitlab/ci/variables/collection_spec.rb

@@ -66,6 +66,14 @@ describe Gitlab::Ci::Variables::Collection do
       expect(collection).to include(key: 'VAR_3', value: '3', public: true)
     end
 
+    it 'does not concatenate resource if it undefined' do
+      collection = described_class.new([{ key: 'VAR_1', value: '1' }])
+
+      collection.concat(nil)
+
+      expect(collection).to be_one
+    end
+
     it 'returns self' do
       expect(subject.concat([key: 'VAR', value: 'test']))
         .to eq subject