Make lineages_topology output optional in FREYJA_UPDATE

[sam-baird]

Currently FREYJA_UPDATE will error with Missing output file(s) for pathogens without a lineages.yml file available in the Freyja-barcodes repo (for example RSV A). This lineage_topology output is not required for the downstream FREYJA_DEMIX process, so this PR just makes the output optional.

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