ag acquire_field fails with cobra_home pfsDesign

ag acquire_field design_id=3161376879636171313 exposure_time=1000 dry_run=yes fit_dscale=no fit_dinr=no visit_id=108080 fails in the SkyCoord.apply_space_motion() call with SkyCoord requires velocity data to evolve the position..

Notably, that designId is for cobra_home, and the GUIDESTARS HDU is structurally OK but empty, of course.

I confirmed with {{ python/agActor/pfs_design.py --design-path=/data/pfsDesign --design-id=3161376879636171313}} that the astropy.sky_coordinates space_motion routine does fail on empty frames, and that it probably still fails in the current astropy.

OK, stepping back, we are trying to do this in order to "run the guide loop" (take exposures with all the standard book-keeping) during times when we might not have guide stars (during twilight, in this case). So maybe we should have a command to do that. Still, 0 guidestars sounds valid, and protecting against this failure is probably worth trying.