Use safe placholder atlas name if atlas XML not found

[tieneupin]

The search for an atlas XML file currently fails if no search results were returned for the atlas:

src/murfey/client/context.py, L97-99

atlas_xml_path = list(
    (source_visit_dir / partial_path).parent.glob("Atlas_*.xml")
)[0]

This PR adjusts that logic so that it proceeds with the post to the backend server with the minimum amount of informaiton present and safe-ish placeholder name for the atlas.

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.58%. Comparing base (0df33b0) to head (a501c5e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #734      +/-   ##
==========================================
- Coverage   46.59%   46.58%   -0.01%     
==========================================
  Files          91       91              
  Lines        9634     9636       +2     
  Branches     1262     1263       +1     
==========================================
  Hits         4489     4489              
- Misses       4932     4933       +1     
- Partials      213      214       +1     

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