[24.0] Fix subwofklow tags serialization type

[arash77]

When exporting a workflow that contains a sub-workflow, the export shows an empty string instead of a list of sub-workflow tags.
Right now, this fixes the typing error (tags should be a list, not a string). However, if we had some tags in the subworkflow they are never serialized, so it will always be an empty list. I think the same happens for subworkflow annotations. Is that something worth fixing? or is there no use for tags or annotations to be embedded in the sub-workflow?

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[mvdbeek]

Nice fix, thank you!

[github-actions]

This PR was merged without a "kind/" label, please correct.