CH-297 Handle missing registry Secrets when generating workflows

[afonsobspinto]

Closes CH-297.

Implemented solution

Handle missing, null or empty registry Secret configuration without crashing. Add imagePullSecrets to the workflow only when a Secret name exists; preserve valid named references.

This enables local/test workflow generation when images are already available or accessible without an explicit pull Secret. Deployments with a valid registry Secret already work.

How to test this PR

From the repository root:

conda activate ch
cd libraries/cloudharness-common
python -m pytest -q tests/test_workflow.py -k 'registry_secret or image_pull_secret'

These nine tests run without Kubernetes. Check that:

• Missing registry.secret, null, {}, an empty string and empty/null names are treated as no configured Secret.
• Rendering without a Secret succeeds and omits spec.imagePullSecrets, rather than generating an empty reference.
• With registry.secret.name: private-registry, rendering retains exactly imagePullSecrets: [{name: private-registry}].

Sanity checks:

• The pull request is explicitly linked to the relevant issue(s)
• The issue is well described: clearly states the problem and the general proposed solution(s)
• In this PR it is explicitly stated how to test the current change
• The issue sets the scope and the type of issue (bug, story, task, etc.)
• The relevant components are indicated in the issue (if any)
• All the automated test checks are passing
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Breaking changes (select one):

• The present changes preserve the preexisting API and behavior
• This PR and the issue are tagged as a breaking-change and the migration procedure is described above

Possible deployment updates issues (select one):

• There is no reason why existing deployments should break after this update
• This PR and the issue are tagged as alert:deployment

Test coverage (select one):

• Tests for the relevant cases are included in this PR
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Documentation (select one):

• The documentation has been updated to match the current changes
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Nice to have (if relevant):

• Screenshots of the changes
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