checkm/qa: Improve input validation

[dialvarezs]

This PR improves the validation of the input coverage_file to handle cases where .isFile() is not defined, for example an empty list ([]).

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the module conventions in the contribution docs
• If necessary, include test data in your PR.
• Remove all TODO statements.
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• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
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    • nf-core modules test <MODULE> --profile docker
    • nf-core modules test <MODULE> --profile singularity
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    • nf-core subworkflows test <SUBWORKFLOW> --profile docker
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