[python] Use new shape in one more spot

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

Note that the intended Python and R API changes are all agreed on and finalized as described in #2407.

Changes:

Found a spot in one-off/ad-hoc testing in my laptop on a particular old dataset.

Notes for Reviewer:

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 85.20%. Comparing base (3d83782) to head (230bdd9).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3321      +/-   ##
==========================================
+ Coverage   85.19%   85.20%   +0.01%     
==========================================
  Files          53       53              
  Lines        5565     5577      +12     
==========================================
+ Hits         4741     4752      +11     
- Misses        824      825       +1     

Flag	Coverage Δ
python	85.20% <28.57%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	85.20% <28.57%> (+0.01%)	⬆️
libtiledbsoma	∅ <ø> (∅)