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:

image: ruby:2.2

test:
  variables:
  script:
     - rspec

What are the relevant issue numbers?

Closes #18764 (closed)
Follow up discussion in: #18775 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Squashed related commits together