Give informative error when empty tsv column

[jfy133]

To close #626

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 - add to the software_versions process and a regex to scrape_software_versions.py
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/eager branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core lint .).
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[github-actions]

Hi @jfy133,

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

Thanks again for your contribution!