Fix sequence validation to handle NaN values in DRAMP database download

[hindrek]

Fixes #537.

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!
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• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
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[github-actions]

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---

Hi @hindrek,

It looks like this pull-request is has been made against the hindrek/funcscan main branch.
The main branch on nf-core repositories should always contain code from the latest release.
Because of this, PRs to main are only allowed if they come from the hindrek/funcscan dev branch.

You do not need to close this PR, you can change the target branch to dev by clicking the "Edit" button at the top of this page.
Note that even after this, the test will continue to show as failing until you push a new commit.

Thanks again for your contribution!

[jfy133]

Thank you very much @hindrek ! DRAMP is being the bane of my life at the moment 🙃

Simple enough fix, so it's ready to go - please however add an entry in the CHANGELOG and add yourself to the contributors list in README and the nextflow.config manifest section :), then I will merge

[jfy133]

Thanks @hindrek