recording: add clip only if information exists

[PhilippKilian]

Fixes following bug:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/celery/app/trace.py", line 453, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/celery/app/trace.py", line 736, in __protected_call__
    return self.run(*args, **kwargs)
  File "/usr/src/app/rest/task/b3lb.py", line 44, in recording_render_record
    return render_record(RecordSet.objects.get(uuid=record_set_uuid))
  File "/usr/src/app/rest/task/recording.py", line 87, in render_record
    record_id = render_by_profile(record_set, record_profile, tempdir)
  File "/usr/src/app/rest/task/recording.py", line 46, in render_by_profile
    render_xges(f"{tempdir}/in/", f"{tempdir}/out/video.xges", record_profile)
  File "/usr/src/app/rest/b3lb/make_xges.py", line 350, in render_xges
    p = Presentation(opts)
  File "/usr/src/app/rest/b3lb/make_xges.py", line 90, in __init__
    self.add_slides()
  File "/usr/src/app/rest/b3lb/make_xges.py", line 263, in add_slides
    info = [i.data for i in slide_time.at(pos.start)][0]
IndexError: list index out of range