fix __str__ for wcs with a None transform

[braingram]

Constructing a WCS without a transform leads to an error for str(wcs):

w = gwcs.WCS(output_frame="icrs")
print(w)

File ~/projects/src/gwcs/gwcs/wcs.py:1347, in WCS.__str__(self)
   1344 for item in self._pipeline[: -1]:
   1345     #model = item[1]
   1346     model = item.transform
-> 1347     if model.name is not None:
   1348         col2.append(model.name)
   1349     else:

AttributeError: 'NoneType' object has no attribute 'name'

This PR updates WCS.__str__ to handle this case so the output is now:

  From   Transform
-------- ---------
detector      None
    icrs      None

The issue was found due to usage of a wcs without a transform in roman_datamodels maker_utils combined with an update to asdf.info that uses the str representation of objects: asdf-format/asdf#1687

This PR includes a test for a wcs without a transform. It appears that WCS.__str__ is otherwise untested so it might be an improvement to add other wcs configurations to the new test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eb9d316) 87.28% compared to head (f13d75b) 87.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #489   +/-   ##
=======================================
  Coverage   87.28%   87.28%           
=======================================
  Files          22       22           
  Lines        3821     3823    +2     
=======================================
+ Hits         3335     3337    +2     
  Misses        486      486           

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[nden]

Th issue of using a string for the coordinate frame comes back again and again. I was hesitant to keep supporting it but it's too convenient for testing. So I'll approve it.

[nden]

Could you add a change log

[braingram]

Thanks for the review. Is the changelog in f13d75b not showing up? I'm seeing it on my end (and appears to be picked up by the CI).

[nden]

Oops, not sure how I missed that. Thanks!