fix: Casting Struct to String panics if n_chunks > 1

[lukemanley]

Fixes #21650

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.48%. Comparing base (43989f8) to head (b62abee).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21656      +/-   ##
==========================================
- Coverage   80.49%   80.48%   -0.01%     
==========================================
  Files        1605     1605              
  Lines      231834   231832       -2     
  Branches     2678     2678              
==========================================
- Hits       186604   186585      -19     
- Misses      44602    44619      +17     
  Partials      628      628              

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