fix warning visium hd when default value was set

[LucaMarconato]

Closes #350

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.07%. Comparing base (23bcb72) to head (d9e8cb7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #353   +/-   ##
=======================================
  Coverage   55.07%   55.07%           
=======================================
  Files          26       26           
  Lines        2809     2809           
=======================================
  Hits         1547     1547           
  Misses       1262     1262           

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/visium_hd.py	75.08% <100.00%> (ø)

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