yokogawa-dlm: silence compiler warning (passing an empty array)

yokogawa-dlm: silence compiler warning (passing an empty array)

Rephrase the config_list() routine such that it does not attempt to
access the first item in an empty array. Be explicit about the digital
channel group's devopts being empty by design, add a comment.

The compiler warning could have been a false positive, but phrasing the
source code differently doesn't harm either, eliminates uncertainty.