Removes presence validation from cache config

lib/gitlab/ci/config/entry/cache.rb

@@ -17,8 +17,6 @@ module Gitlab
             MULTIPLE_CACHE_LIMIT = 4
 
             validations do
-              validates :config, presence: true
-
               validate do
                 unless config.is_a?(Hash) || config.is_a?(Array)
                   errors.add(:config, 'can only be a Hash or an Array')

spec/lib/gitlab/ci/config/entry/cache_spec.rb

@@ -13,6 +13,14 @@ RSpec.describe Gitlab::Ci::Config::Entry::Cache do
     end
 
     describe '#valid?' do
+      context 'with an empty hash as cache' do
+        let(:config) { {} }
+
+        it 'is valid' do
+          expect(entry).to be_valid
+        end
+      end
+
       context 'when configuration is valid with a single cache' do
         let(:config)  { { key: 'key', paths: ["logs/"], untracked: true } }