Handle nil services error in external pipeline validation

Adding a blank `services` entry would previously cause an error. We now
trim nil values when building the payload for the Pipeline Validation
Service.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/348631

Changelog: fixed

lib/gitlab/ci/pipeline/chain/validate/external.rb

@@ -113,7 +113,7 @@ module Gitlab
                 name: build[:name],
                 stage: build[:stage],
                 image: build.dig(:options, :image, :name),
-                services: build.dig(:options, :services)&.map { |service| service[:name] },
+                services: service_names(build),
                 script: [
                   build.dig(:options, :before_script),
                   build.dig(:options, :script),
@@ -122,6 +122,14 @@ module Gitlab
               }
             end
 
+            def service_names(build)
+              services = build.dig(:options, :services)
+
+              return unless services
+
+              services.compact.map { |service| service[:name] }
+            end
+
             def stages_attributes
               command.yaml_processor_result.stages_attributes
             end

spec/lib/gitlab/ci/pipeline/chain/validate/external_spec.rb

@@ -24,6 +24,7 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Validate::External do
     second_stage_job_name:
       stage: second_stage
       services:
+        -
         - postgres
       before_script:
         - echo 'first hello'
@@ -142,6 +143,23 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Validate::External do
 
         perform!
       end
+
+      it 'returns expected payload' do
+        expect(::Gitlab::HTTP).to receive(:post) do |_url, params|
+          payload = Gitlab::Json.parse(params[:body])
+
+          builds = payload['builds']
+          expect(builds.count).to eq(2)
+          expect(builds[0]['services']).to be_nil
+          expect(builds[0]['stage']).to eq('first_stage')
+          expect(builds[0]['image']).to eq('hello_world')
+          expect(builds[1]['services']).to eq(['postgres'])
+          expect(builds[1]['stage']).to eq('second_stage')
+          expect(builds[1]['image']).to be_nil
+        end
+
+        perform!
+      end
     end
 
     context 'when EXTERNAL_VALIDATION_SERVICE_TOKEN is set' do