fix: fix code for empty message

[flying-sheep]

the test code so far generated

.. version-deprecated:: foo
   None

which is certainly not intended

[ilia-kats]

Thanks, missed that part when converting from a dataclass to the string subclass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (a77c3d6) to head (3cdecae).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   97.43%   97.50%   +0.06%     
==========================================
  Files           3        3              
  Lines         117      120       +3     
==========================================
+ Hits          114      117       +3     
  Misses          3        3              

Files with missing lines	Coverage Δ
src/scverse_misc/_deprecated.py	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.