fix: check whether contrastsheet param is null

[kelly-sovacool]

Changes

Remove redundant contrasts param, instead just set contrastsheet to null by default and check whether it is null before trying to create the file/path channel.

Issues

• resolves change {params.contrast} to be TRUE/FALSE rather than input file #196
• partly undoes some of the changes from Improvements to chipseeker, contrast & input checks, docs #192, which was merged prematurely

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• [ ] Write unit tests for any new features, bug fixes, or other code changes. testing framework not yet implemented
• [ ] Update docs if there are any API changes. on hold until before public release
• [ ] If a new nextflow process is implemented, define the process container and stub.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/