Fix downstream pipeline failure on nil variable value

What does this MR do and why?

Gitlab::Ci::Variables::Collection::Item accepts a variable value that is either a String or nil, and a nil value is reachable in practice: the pipelines API marks value as optional, so triggering a pipeline with { "key": "MY_VAR", "value": null } persists it as nil.

This MR returns the value untouched when it is nil. There is nothing to expand, and forwarding nil verbatim matches what RawVariableGenerator already does for raw: true variables.

How to set up and validate locally

1. Ensure your project does not restrict pipeline variables

2. In your project use this configuration to get a parent-child-pipeline:

   # --- Parent pipeline -------------------------------------------------------
   
   trigger-downstream:
     stage: test
     rules:
       - if: $CI_PIPELINE_SOURCE != "parent_pipeline"
     trigger:
       include: .gitlab-ci.yml
       forward:
         pipeline_variables: true
       strategy: depend
   
   # --- Child pipeline --------------------------------------------------------
   
   show-forwarded-variable:
     stage: test
     rules:
       - if: $CI_PIPELINE_SOURCE == "parent_pipeline"
     script:
       - echo "MY_VAR is >>>${MY_VAR}<<<"

3. Trigger the parent pipeline through the API with a null-valued variable:

   curl --request POST \
     --header "PRIVATE-TOKEN: $TOKEN" \
     --header "Content-Type: application/json" \
     --data '{ "ref": "main", "variables": [{ "key": "MY_VAR", "value": null }] }' \
     "https://gitlab.com/api/v4/projects/<PROJECT_ID>/pipeline"

4. Before this change, the bridge fails and Ci::CreateDownstreamPipelineWorker raises NoMethodError. After it, the downstream pipeline is created and MY_VAR is forwarded.

References

• Resolves #603635 (closed)