Handle untargeted ACDCs in IPEX Grant artifact gathering

[kentbull]

Fixes #452 crash on untargeted ACDC or missing issuee

Skip issuee KEL and delegation artifact gathering when an ACDC has no disclosed issuee, including compact attribute sections. Issuer, delegated issuer/issuee, registry, and credential TEL gathering remain intact.

Adds direct unit coverage for every branch of gatherArtifacts() and verifies the existing SignifyPy untargeted-IPEX flow end to end.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.44%. Comparing base (ba5001d) to head (b5d8e64).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
+ Coverage   88.24%   88.44%   +0.19%     
==========================================
  Files          26       26              
  Lines        5931     5938       +7     
==========================================
+ Hits         5234     5252      +18     
+ Misses        697      686      -11     

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[iFergal]

LGTM otherwise!