Fix null validation

[jfy133]

To address #723 and also a bit of code cleanup for consistent (following @ewels suggestion to replace == '' and isEmpty() like conditions with null)

⚠️ need to merge #742 first!

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).