Fixed missing MultiQC when quast or busco is skipped

[skrakau]

The multiqc process required as input the output from quast and busco, i.e. no MultiQC report was generated when --skip_quast or --skip_busco was specified. Added .ifEmpty([]) to fix this.

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

[d4straub]

Looks good!
Actually, a comment in the change-log would be good as well.

[skrakau]

@d4straub thanks, done!