Ensure nil User-Agent doesn't break the CI API

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/25895-fix-headers-in-ci-api-helpers.yml

+---
+title: Ensure nil User-Agent doesn't break the CI API
+merge_request:
+author:

lib/ci/api/helpers.rb

@@ -60,7 +60,7 @@ module Ci
       end
 
       def build_not_found!
-        if headers['User-Agent'].match(/gitlab-ci-multi-runner \d+\.\d+\.\d+(~beta\.\d+\.g[0-9a-f]+)? /)
+        if headers['User-Agent'].to_s.match(/gitlab-ci-multi-runner \d+\.\d+\.\d+(~beta\.\d+\.g[0-9a-f]+)? /)
           no_content!
         else
           not_found!

spec/requests/ci/api/builds_spec.rb

@@ -37,6 +37,11 @@ describe Ci::API::Builds do
           let(:user_agent) { 'Go-http-client/1.1' }
           it { expect(response).to have_http_status(404) }
         end
+
+        context "when runner doesn't have a User-Agent" do
+          let(:user_agent) { nil }
+          it { expect(response).to have_http_status(404) }
+        end
       end
 
       context 'when there is a pending build' do