chore(EC-2153): adapt digest update script for pipeline-level default

[jsmid1]

DO NOT MERGE BEFORE conforma/cli#3573 IS MERGED

The POLICY_BUNDLE_DIGEST default has been moved from task definitions to the enterprise-contract pipeline in conforma/cli (EC-2153). Task definitions now have an empty default.

Changes:

• update-policy-digest.sh: generalize variable names and exit cleanly when no files carry a non-empty POLICY_BUNDLE_DIGEST default
• konflux-policy.yaml: mark the digest update step as expected no-op since extracted tasks no longer carry the default
• spec/update_policy_digest_spec.sh: add tests for Pipeline YAML, empty defaults, and the no-files-found case

Resolves: EC-2153

[coderabbitai]

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