Obscure WCSLIB error when trying to deep copy 0-d WCS

[astrofrog]

Description

When trying to deep copy a 0-d WCS, an obscure error occurs. We should either disallow 0-d WCSes from being created in .sub, or gracefully deep copy the 0-d WCS.

Expected behavior

The deep copy should work or the .sub call should fail because it results in a 0-d WCS.

How to Reproduce

In [1]: from astropy.wcs import WCS, WCSSUB_SPECTRAL

In [2]: wcs = WCS(naxis=2)

In [3]: wcs_sub = wcs.sub([WCSSUB_SPECTRAL])

In [4]: wcs_sub.deepcopy()
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
Cell In[4], line 1
----> 1 wcs_sub.deepcopy()

File ~/python/dev/lib/python3.11/site-packages/astropy/wcs/wcs.py:653, in WCS.deepcopy(self)
    646 def deepcopy(self):
    647     """
    648     Return a deep copy of the object.
    649 
    650     Convenience method so user doesn't have to import the
    651     :mod:`copy` stdlib module.
    652     """
--> 653     return copy.deepcopy(self)

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/copy.py:153, in deepcopy(x, memo, _nil)
    151 copier = getattr(x, "__deepcopy__", None)
    152 if copier is not None:
--> 153     y = copier(memo)
    154 else:
    155     reductor = dispatch_table.get(cls)

File ~/python/dev/lib/python3.11/site-packages/astropy/wcs/wcs.py:626, in WCS.__deepcopy__(self, memo)
    620 new_copy = self.__class__()
    621 new_copy.naxis = deepcopy(self.naxis, memo)
    622 WCSBase.__init__(
    623     new_copy,
    624     deepcopy(self.sip, memo),
    625     (deepcopy(self.cpdis1, memo), deepcopy(self.cpdis2, memo)),
--> 626     deepcopy(self.wcs, memo),
    627     (deepcopy(self.det2im1, memo), deepcopy(self.det2im2, memo)),
    628 )
    629 for key, val in self.__dict__.items():
    630     new_copy.__dict__[key] = deepcopy(val, memo)

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/copy.py:153, in deepcopy(x, memo, _nil)
    151 copier = getattr(x, "__deepcopy__", None)
    152 if copier is not None:
--> 153     y = copier(memo)
    154 else:
    155     reductor = dispatch_table.get(cls)

RuntimeError: NULL error object in wcslib

Versions

macOS-14.1.1-arm64-arm-64bit
Python 3.11.2 (main, Mar 23 2023, 23:28:33) [Clang 14.0.0 (clang-1400.0.29.202)]
astropy 7.0.0.dev14+gaae4425033.d20240411
Numpy 1.26.4
pyerfa 2.0.1.1
Scipy 1.10.1
Matplotlib 3.6.3