[minor] Better handling of skipping ERCC process

[jackkamm]

The quantifyERCCs step should only happen when doing MSSPE sequencing, not ARTIC sequencing.

Currently, we de facto skip it for ARTIC -- in particular, we still run the process, but just create a dummy output. However it would be better to skip the process entirely.

A slight complication is that one of the downstream processes (computeStats) consumes the output of quantifyERCCs, which is why we created the dummy output instead of skipping the process entirely.