Barycentric Corrections fail when both TARGTRA and RA are unassigned

Barycentric Correction Fails for some old exposures where the header keys aren't properly labeled. This is a fringe case that hasn't appeared in the most recent month of data.

```
RUNNING: desi_extract_spectra -w 7520.0,9824.0,0.8 -i /global/cfs/cdirs/desi/spectro/redux/daily/preproc/20200122/00043028\
/preproc-z7-00043028.fits -p /global/cfs/cdirs/desi/spectro/redux/daily/exposures/20200122/00043028/psf-z7-00043028.fits -\
o /global/cfs/cdirs/desi/spectro/redux/daily/exposures/20200122/00043028/frame-z7-00043028.fits --psferr 0.1 --barycentric\
-correction
Traceback (most recent call last):
File "/global/homes/k/kremin//workspace/most_recent_calib/desispec/bin/desi_proc", line 664, in
desispec.scripts.extract.main_mpi(extract_args, comm=comm_extract)
File "/global/homes/k/kremin/workspace/most_recent_calib/desispec/py/desispec/scripts/extract.py", line 337, in main_mpi
barycentric_correction_factor = barycentric_correction_multiplicative_factor(img.meta)
File "/global/homes/k/kremin/workspace/most_recent_calib/desispec/py/desispec/scripts/extract.py", line 95, in barycentr\
ic_correction_multiplicative_factor
raise KeyError("no TARGTRA nor RA in header")
KeyError: 'no TARGTRA nor RA in header'
```

Start in py/desispec/scripts/extract.py, especially barycentric_correction_multiplicative_factor and its call from main_mpi. Reproduce the reported desi_extract_spectra case with headers lacking both TARGTRA and RA, then inspect nearby tests or metadata handling to establish the expected behavior for these old exposures and verify that behavior.

Written by the indexing model from the issue text.