PyGRB: minor fixes to workflow generator

[sebastiangomezlopez]

Standard information about the request

This is a: bug fix.

This change affects: PyGRB

This change changes: PyGRB workflow generation.

Motivation

This PR contains 2 separate bugfixes uncovered while testing the code on single detector GRB runs using O4a data.

Contents

1. In the context of handling Veto definer files, pycbc_make_offline_grb_workflow checks for veto segments present in the onsource timewindow, i.e. around the GRB trigger, and throws an error if that's the case to avoid submitting jobs and end up wasting CPU hours.

Currently lines 252-264 lack an if statement to avoid operating with an empty list, when avail_segs has no elements.

2. Single detector runs were not tested in the past. However by recently doing so, we uncovered a minor bug in pycbc/workflow/jobsetup.py. Specifically lines 690&691 were mistakenly using the method get_opts from the wrong object.