Avoid exceptions from un-symbolizable job names

If `true` or `false` is used as a job name, it is interpreted by the
YAML parser as a boolean, which does not have the `to_sym` method. This
caused 500s when linting

Changelog: fixed

doc/ci/jobs/index.md

@@ -95,6 +95,9 @@ You can't use these keywords as job names:
 - `variables`
 - `cache`
 - `include`
+- `true`
+- `false`
+- `nil`
 
 Job names must be 255 characters or less. [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/342800)
 in GitLab 14.5, [with a feature flag](../../administration/feature_flags.md) named `ci_validate_job_length`.

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

@@ -15,7 +15,7 @@ module Gitlab
 
             validate do
               each_unmatched_job do |name|
-                errors.add(name, 'config should implement a script: or a trigger: keyword')
+                errors.add(name.to_s, 'config should implement a script: or a trigger: keyword')
               end
 
               unless has_visible_job?

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

@@ -70,6 +70,14 @@ RSpec.describe Gitlab::Ci::Config::Entry::Jobs do
           it 'reports error' do
             expect(entry.errors).to include 'jobs rspec config should implement a script: or a trigger: keyword'
           end
+
+          context 'when the job name cannot be cast directly to a symbol' do
+            let(:config) { { true => nil } }
+
+            it 'properly parses the job name without raising a NoMethodError' do
+              expect(entry.errors).to include 'jobs true config should implement a script: or a trigger: keyword'
+            end
+          end
         end
 
         context 'when no visible jobs present' do