Unexpected failure in spectro completeness notebook

In `onskydata/spectro_completeness.ipynb`, the cell that begins
```
z = []
fba = []
exp_fba = []
target = []
zmtl = []
```
throws an exception when run on Perlmutter with kernel 22.2 or 23.1. The full output from the cell is:
```
failed for redrock file from petal 1, tileid 4117 and night 20221122
failed for redrock file from petal 5, tileid 4117 and night 20221122
failed for zmtl file from petal 1, tileid 4117 and night 20221122
failed for zmtl file from petal 5, tileid 4117 and night 20221122
failed for redrock file from petal 1, tileid 5648 and night 20220324
failed for redrock file from petal 7, tileid 8776 and night 20220104
failed for zmtl file from petal 7, tileid 8776 and night 20220104
failed for redrock file from petal 7, tileid 8777 and night 20220104
failed for zmtl file from petal 7, tileid 8777 and night 20220104
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[6], line 45
42 print("failed for zmtl file from petal {}, tileid {} and night {}".format(petal, tileid, lastnight))
44 data_z = np.hstack(z)
---> 45 data_fba = np.hstack(fba)
46 data_exp_fba = np.hstack(exp_fba)
47 data_target = np.hstack(target)

File <__array_function__ internals>:180, in hstack(*args, **kwargs)

File /global/common/software/desi/perlmutter/desiconda/20230111-2.1.0/conda/lib/python3.10/site-packages/numpy/core/shape_base.py:343, in hstack(tup)
341 # As a special case, dimension 0 of 1-dimensional arrays is "horizontal"
342 if arrs and arrs[0].ndim == 1:
--> 343 return _nx.concatenate(arrs, 0)
344 else:
345 return _nx.concatenate(arrs, 1)

File <__array_function__ internals>:180, in concatenate(*args, **kwargs)

TypeError: invalid type promotion with structured datatype(s).
```