conditinalize in event that yld[si] is zero to avoid divide by zero err

[dhoogest]

addresses #75

$ ./nextflow ./main.nf -params-file params.json test passing following change.

[crosenth]

Added step to drop empty raw fastqs before barcodecop but note they will still be processed and counted in read_manifest

[crosenth]

https://github.com/nhoffman/dada2-nf/issues/75