Preserve IdentifiableArtefact(id="")

[khaeru]

@glatterf42 reports that here downstream tests fail with sdmx 2.19.0, but not sdmx 2.18.0.

This happens when an IdentifiableArtefact is initialized with an empty string ("") as its ID. Previously, this value was preserved. As a consequence of changes in #198, empty string was replaced with MissingID. There was no test covering this case.

This PR adds the missing test and a fix.

PR checklist

• Checks all ✅
• Update documentation N/A, bugfix
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.96%. Comparing base (dd877b3) to head (ac290a6).
Report is 96 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
- Coverage   98.90%   97.96%   -0.95%     
==========================================
  Files          97       97              
  Lines        8045     8051       +6     
==========================================
- Hits         7957     7887      -70     
- Misses         88      164      +76     

Files with missing lines	Coverage Δ
sdmx/model/common.py	99.90% <100.00%> (ø)
sdmx/tests/model/test_common.py	100.00% <100.00%> (ø)

... and 18 files with indirect coverage changes