Find a better way to provide a fake file of the polymorphism position when none exists to filterAlignment

The BED files does not include any polymorphism, this information can indeed be missing in sequences from the literature. The filterAlignment requires this input if the user wants to filter on the position/value of the polymorphism. For now, a fake file has been created to be passed instead.

in main.nf (if BED):

ch_no_poly_file = Channel.fromPath('conf/None')
filterAlignment(ch_markers_fasta, alignmentMinimap.out.paf, ch_no_poly_file, extra_opt)

in filter_alignement.py

if args.polymorphism_markers and args.polymorphism_markers != 'None': # nextflow optional input

There must exist a better to provide an optional input (this should be improved in future updates according to Nextflow)