Return untouched hash if no job is parallelized

lib/gitlab/ci/config/normalizer.rb

@@ -9,14 +9,16 @@ module Gitlab
         end
 
         def normalize_jobs
-          extract_parallelized_jobs
+          extract_parallelized_jobs!
+          return @jobs_config if @parallelized_jobs.empty?
+
           parallelized_config = parallelize_jobs
           parallelize_dependencies(parallelized_config)
         end
 
         private
 
-        def extract_parallelized_jobs
+        def extract_parallelized_jobs!
           @parallelized_jobs = {}
 
           @jobs_config.each do |job_name, config|

spec/lib/gitlab/ci/config/normalizer_spec.rb

@@ -31,6 +31,14 @@ describe Gitlab::Ci::Config::Normalizer do
       expect(configs).to all(eq(original_config))
     end
 
+    context 'when the job is not parallelized' do
+      let(:job_config) { { script: 'rspec', name: 'rspec' } }
+
+      it 'returns the same hash' do
+        is_expected.to eq(config)
+      end
+    end
+
     context 'when there is a job with a slash in it' do
       let(:job_name) { :"rspec 35/2" }