fix str column showing object dtype

[melonora]

The table validation currently fails when the instance_key column contains str values. This is because when setting the dtype of such a column to str, the dtype is actually object. This PR corrects the check so that an instance_key column having the object dtype and having as first element a str value, would still pass the check.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.58%. Comparing base (c2e4ef9) to head (4101557).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #489   +/-   ##
=======================================
  Coverage   92.58%   92.58%           
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  Files          43       43           
  Lines        5866     5867    +1     
=======================================
+ Hits         5431     5432    +1     
  Misses        435      435           

Files	Coverage Δ
src/spatialdata/models/models.py	88.02% <100.00%> (+0.02%)	⬆️