Fix logging error when no reverse image are provided

[gagnonanthony]

When trying to run the pipeline with --skip_dwi_preprocessing without submitting a reverse b0 image, the subworkflow loading the data returned an error. Changed the assertion to omit if --skip_dwi_preprocessing is selected. Tests should catch it now.

It will be removed when we switch to mandatory BIDS input.

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 pipeline conventions in the contribution docs
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• 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).