Print workflow output variables only when they contain results

New feature

Allow printing of workflow output variables (PR #6875) only when they contain results, to avoid displaying empty variables in the final logs.

Use case

In a Nextflow workflow using a switch/case process structure, some branches may not produce results depending on the input parameters. Currently, all output variables are printed in the final logs, including empty ones (e.g., afMassiveAlphaFillpredictions:, afMassiveAlphaFillMultiQC:).

This makes the final output less readable and less informative, as users must manually filter out empty variables to identify relevant results.

Outputs:

  /[...]/results_workflow_output

afMassiveAlphaFillpredictions:

afMassiveAlphaFillMultiQC:

colabFoldSearchResult:
  - [BTB-domain, colabFoldSearch/BTB-domain]

colabFoldSearchVersions:
  - colabFoldSearch/versions.txt

colabFoldSearchOptions:
  - colabFoldSearch/options.txt

Outputs:

  /[...]/results_workflow_output

colabFoldSearchResult:
  - [BTB-domain, colabFoldSearch/BTB-domain]

colabFoldSearchVersions:
  - colabFoldSearch/versions.txt

colabFoldSearchOptions:
  - colabFoldSearch/options.txt

Suggested implementation

Introduce an option or mechanism to:

• Automatically detect if a variable/channel is empty before printing it.
• Skip printing empty variables in the final human-readable output.

Read PR #6875 first, then locate the code that renders workflow output variables in the final human-readable logs. Trace how empty channels or variables are represented and add coverage for a switch/case workflow with unused outputs; done means empty variables are omitted while populated outputs remain unchanged.