Fix/prevent falco to process empty fastq files

[glichtenstein]

PR checklist

What about doing the filter directly on the falco module main.nf. This way the subworkflow/bcl_demultiplex/main.nf does not have to handle this issue nor the workflow/demultiplex.nf
I have used a file size filter, to avoid inserting in the falco list of ${reads} those files that have less than 20bytes.
Rationale: An empty text file compressed in tar.gz is usually 20 bytes in size.

Closes #5638

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