Handle empty mutation cohorts without failing

[Copilot]

When no elements meet the mutation cutoff, the CLI aborts with a UserInputError. Desired behavior is to complete successfully and emit empty results.

• Graceful early exit on empty cohorts: write empty elements/clusters outputs and return without raising.
• Robust empty-result serialization: postprocessing now emits correctly ordered headers for empty element results.

if not element_mutations_cutoff:
    logger.warning('No element found with enough mutations to perform analysis. Writing empty results.')
    sorted_list_elements = postp.write_element_results(
        genome=genome,
        results=({}, {}),
        directory=output_directory,
        file=elements_output_file,
        is_gzip=gzip
    )
    postp.write_cluster_results(
        genome=genome,
        results=({}, {}),
        directory=output_directory,
        file=clusters_output_file,
        sorter=sorted_list_elements,
        is_gzip=gzip
    )
    return