Cannot access `label` property when indexing a single level

[wtbarnes]

Accessing the label property of Levels when indexing a single level is broken.

import fiasco
import astropy.units as u

s_10 = fiasco.Ion('S X', 1*u.MK)
s_10[0].label

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[125], line 2
      1 s_10 = fiasco.Ion('S X', 1*u.MK)
----> 2 s_10[0].label

File [~/Documents/projects/active-region-evolution-study/.venv/lib/python3.12/site-packages/fiasco/levels.py:103](http://localhost:8888/lab/workspaces/auto-A/tree/Documents/projects/active-region-evolution-study/notebooks/~/Documents/projects/active-region-evolution-study/.venv/lib/python3.12/site-packages/fiasco/levels.py#line=102), in Levels.label(self)
     97 @property
     98 def label(self):
     99     """
    100     Label denoting level configuration, multiplicity, angular momentum label,
    101     and total angular momentum.
    102     """
--> 103     zipped = zip(self.configuration,
    104                  self.multiplicity,
    105                  self.orbital_angular_momentum_label,
    106                  self.total_angular_momentum)
    107     return np.array([f"{i} {j}{k}{Fraction(l.value)}" for i,j,k,l in zipped])

TypeError: iteration over a 0-d array

This, however, does work:

s_10[:1].label

This issue seems to be trying to zip together arrays with 0 length. It may be that we just need to special case instances where the properties have length 0.