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