Fixed missing input when --keep_phix is specified

[skrakau]

When --keep_phix is specified, reads were not redirected to channels trimmed_reads_kraken2 and trimmed_reads_bowtie2, which causes errors (when the downstream processes using them are not explicitly turned off). Fixed that.

In case that was on purpose and the parameter --keep_phix should only be used with certain parameter combinations, this should be caught before.

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 necessary, also make a PR on the nf-core/mag branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: https://github.com/nf-core/mag/tree/master/.github/CONTRIBUTING.md

[HadrienG]

Looks good to me!

[d4straub]

Hmhm changelog? ;)