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 Please view this file on the master branch, on stable branches it's out of date.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

@@ -344,13 +344,13 @@ module Ci
         end
       end
     end
-    
+
     describe "Scripts handling" do
       let(:config_data) { YAML.dump(config) }
       let(:config_processor) { GitlabCiYamlProcessor.new(config_data, path) }
-      
+
       subject { config_processor.builds_for_stage_and_ref("test", "master").first }
-      
+
       describe "before_script" do
         context "in global context" do
           let(:config) do
@@ -359,12 +359,12 @@ module Ci
               test: { script: ["script"] }
             }
           end
-          
+
           it "return commands with scripts concencaced" do
             expect(subject[:commands]).to eq("global script\nscript")
           end
         end
- 
+
         context "overwritten in local context" do
           let(:config) do
             {
@@ -522,19 +522,41 @@ module Ci
         end
 
         context 'when syntax is incorrect' do
-          it 'raises error' do
-            variables = [:KEY1, 'value1', :KEY2, 'value2']
-
-            config =  YAML.dump(
-              { before_script: ['pwd'],
-                rspec: {
-                  variables: variables,
-                  script: 'rspec' }
-              })
+          context 'when variables defined but invalid' do
+            it 'raises error' do
+              variables = [:KEY1, 'value1', :KEY2, 'value2']
+
+              config =  YAML.dump(
+                { before_script: ['pwd'],
+                  rspec: {
+                    variables: variables,
+                    script: 'rspec' }
+                })
+
+              expect { GitlabCiYamlProcessor.new(config, path) }
+                .to raise_error(GitlabCiYamlProcessor::ValidationError,
+                                 /job: variables should be a map/)
+            end
+          end
 
-            expect { GitlabCiYamlProcessor.new(config, path) }
-              .to raise_error(GitlabCiYamlProcessor::ValidationError,
-                               /job: variables should be a map/)
+          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