Merge branch 'fix/error-when-job-variables-not-defined-but-specified' into 'master'

Fix error when CI job variables key used but not specified

## What does this MR do?

This MR fixes a an error when CI job variables specified, but not defined:

```yaml
image: ruby:2.2

test:
  variables:
  script:
     - rspec
```

## What are the relevant issue numbers?

Closes #18764  
Follow up discussion in: #18775 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4745

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.9.0 (unreleased)
+  - Fix error when CI job variables key specified but not defined
   - Fix pipeline status when there are no builds in pipeline
   - Fix Error 500 when using closes_issues API with an external issue tracker
   - Add more information into RSS feed for issues (Alexander Matyushentsev)

lib/ci/gitlab_ci_yaml_processor.rb

@@ -54,7 +54,7 @@ module Ci
       job = @jobs[name.to_sym]
       return [] unless job
 
-      job.fetch(:variables, [])
+      job[:variables] || []
     end
 
     private

spec/lib/ci/gitlab_ci_yaml_processor_spec.rb

@@ -522,6 +522,7 @@ module Ci
         end
 
         context 'when syntax is incorrect' do
+          context 'when variables defined but invalid' do
             it 'raises error' do
               variables = [:KEY1, 'value1', :KEY2, 'value2']
 
@@ -537,6 +538,27 @@ module Ci
                                  /job: variables should be a map/)
             end
           end
+
+          context 'when variables key defined but value not specified' do
+            it 'returns empty array' do
+              config =  YAML.dump(
+                { before_script: ['pwd'],
+                  rspec: {
+                    variables: nil,
+                    script: 'rspec' }
+                })
+
+              config_processor = GitlabCiYamlProcessor.new(config, path)
+
+              ##
+              #  TODO, in next version of CI configuration processor this
+              # should be invalid configuration, see #18775 and #15060
+              #
+              expect(config_processor.job_variables(:rspec))
+                .to be_an_instance_of(Array).and be_empty
+            end
+          end
+        end
       end
 
       context 'when job variables are not defined' do