Fix usage of empty versions channel from trimming

[pinin4fjords]

For proper explanation of this, see previous fix: nf-core/rnaseq#1317

TL;DR is that we don't need ifEmpty(). Also, fastq_qc_trim_filter_setstrandedness shouldn't even be running the trimming subworkflows if skip_trimming is true.

PR checklist

Closes #XXX

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