Allow to store null variables

lib/gitlab/ci/variables/collection/item.rb

@@ -6,8 +6,8 @@ module Gitlab
       class Collection
         class Item
           def initialize(key:, value:, public: true, file: false)
-            raise ArgumentError, "`#{key}` must be of type String, while it was: #{value.class}" unless
-              value.is_a?(String)
+            raise ArgumentError, "`#{key}` must be of type String or nil value, while it was: #{value.class}" unless
+              value.is_a?(String) || value.nil?
 
             @variable = {
               key: key, value: value, public: public, file: file

spec/lib/gitlab/ci/variables/collection/item_spec.rb

@@ -36,7 +36,7 @@ describe Gitlab::Ci::Variables::Collection::Item do
     shared_examples 'raises error for invalid type' do
       it do
         expect { described_class.new(key: variable_key, value: variable_value) }
-          .to raise_error ArgumentError, /`#{variable_key}` must be of type String, while it was:/
+          .to raise_error ArgumentError, /`#{variable_key}` must be of type String or nil value, while it was:/
       end
     end
 
@@ -46,7 +46,7 @@ describe Gitlab::Ci::Variables::Collection::Item do
       let(:variable_value) { nil }
       let(:expected_value) { nil }
 
-      it_behaves_like 'raises error for invalid type'
+      it_behaves_like 'creates variable'
     end
 
     context "when it's an empty string" do